Topic
Underground questions for kids
Here are 219 questions about what lies underground from 19 A Million Whys books, each with a short video answer. Book 24, The Underground Expedition is all about what lies underground, and the rest come from other books in the series. Tap any question to watch the video and read the answer.
From Book 24: The Underground Expedition (150)
What would you find if you dug a hole straight down?
As you dig, you would pass through layers. First comes a thin top layer of dark, crumbly topsoil full of roots and tiny creatures.
What makes topsoil dark and crumbly?
Topsoil is the top layer of soil where plants grow. It is dark because it holds a lot of rotted plant and animal bits mixed with tiny pieces of rock.
What is subsoil, the lighter layer below the topsoil?
Subsoil sits under the topsoil. It has less rotted plant material, so it is lighter in color, often yellow, orange, or tan.
What is bedrock?
Bedrock is the solid rock that lies beneath all the soil and loose stones. It may be only a few inches below the surface, or hundreds of feet down.
What is the deepest hole people have ever drilled?
The Kola Superdeep Borehole in northern Russia reached about 7.6 miles down, after drilling on and off for almost 20 years. That is deeper than the deepest spot in the ocean, which is about 7 miles down.
Why does it get hotter the deeper you go underground?
Earth's center is very hot, and its heat slowly flows outward. As you go deeper, you get closer to that heat.
What is the water table?
The water table is the upper edge of the ground that is completely soaked with water. Rain soaks into the soil and trickles down until it reaches a layer where all the little spaces are filled.
Why does mud get so sticky and slippery?
Mud is soil mixed with water. Clay and silt grains are so tiny that water coats each one in a thin film.
What is the frost line, and why are pipes buried below it?
In cold places, the ground freezes from the top down in winter. The frost line is the deepest point where the ground freezes.
What is a pingo, a hill with an ice heart?
A pingo is a round hill with a core of solid ice, found where the ground stays frozen. It forms when a shallow lake drains.
How can solid-looking ground hold both air and water?
Soil looks solid, but it is full of tiny gaps called pores between its grains, a bit like a sponge. About half of the space in good soil is air and water.
Why is soil different colors in different places?
Soil takes its color from what is in it. Dark brown or black soils have lots of rotted plant material.
Why does frozen ground push rocks up to the surface?
When water in soil freezes, it expands and pushes the soil upward. This is called frost heave.
Why does water sink through sand but sit on top of clay?
Sand grains are large, so the gaps between them are wide and water drains quickly. Clay grains are tiny and flat, packed so tightly that the gaps are very small, and water passes through very slowly.
Where does rainwater go when it soaks into the ground?
Some rain runs along the surface into streams. Some soaks into the soil, where roots take it up or the soil holds it in tiny gaps.
Why does the ground sometimes sink to make a sinkhole?
In some places, the rock under the soil is limestone or gypsum, which slowly dissolves in water. Over time, water can carve out empty spaces underground.
Why is there always leftover dirt when you fill a hole back in?
Soil in the ground is packed tightly. When you dig it up, the dirt becomes fluffy and takes up more space because it has more air in it.
Why can you see stripes in a road cut or riverbank?
The stripes are layers of sediment that built up in different periods. Each layer is made of different materials, such as sand, clay, or pebbles, and the colors reflect the minerals in them.
How long does it take to make an inch of topsoil?
It takes a long time. Rocks break into tiny pieces by weathering, and then dead plants and animals mix in.
What happens to things we bury in the ground?
Soil is full of tiny decomposers, like bacteria and fungi, that break down things that were once alive. A buried apple core can disappear in weeks or months.
How can a jar of muddy water show the layers in soil?
Put some soil in a jar of water, shake it, and let it settle. Sand sinks first because it is heavy, forming a layer on the bottom within a minute.
How deep does life go underground?
Scientists have found tiny living things called microbes in rock more than 2 miles below Earth's surface. They live in cracks and pores, using chemicals from the rocks for energy instead of sunlight.
What is leaf litter, and what happens to it?
Leaf litter is the loose layer of fallen leaves, twigs, and bits of bark on the forest floor. It protects the soil, keeps it moist, and shelters many small animals.
Why does the ground smell so fresh after rain?
The smell has a name: petrichor. When raindrops hit dry soil, they trap tiny air bubbles that burst and send out a mist of smelly particles.
Why is soil deep in some places and thin in others?
Soil depth depends on how fast it forms and how fast it is carried away. On flat land with plenty of plants, soil builds up and gets thick.
How many living things can fit in a spoonful of soil?
A spoonful of healthy soil can hold billions of bacteria, long threads of fungi, and thousands of other tiny creatures such as mites, microscopic worms, and springtails. Most of them are too small to see without a microscope.
What is inside an ant nest?
An ant nest is a maze of tunnels and rooms underground. There are chambers for the queen, for eggs and larvae, for storing food, and even for waste.
How do ants get through the winter underground?
When it gets cold, ants move deeper in their nests, below the frost line where the soil stays warmer. They slow down, gather in clusters, and eat food stored earlier or live off body fat.
Why do some cicadas spend 13 or 17 years underground?
Periodical cicada nymphs live in the soil, feeding on sap from tree roots, for either 13 or 17 years. Then, all in the same year, they climb out, shed their skins, and become noisy adults.
What are grubs, and what do they become?
A grub is a young beetle that looks like a fat, curved, creamy-white worm with a brown head and six small legs. Grubs live in the soil and munch on roots and decaying plants.
How does a worm push through hard soil without legs?
An earthworm has two sets of muscles. One squeezes it long and thin, and the other makes it short and fat.
How can a worm sense the world without eyes?
Worms have no eyes, but their skin has special cells that sense light, so they can tell dark from bright. They also feel vibrations, touch, and chemicals.
What are nematodes, and where do they live?
Nematodes are tiny roundworms, mostly smaller than a grain of salt. A handful of soil can contain thousands of them.
How do springtails jump when they have no wings?
A springtail has a little tail-like part tucked under its belly, held in place like a spring. When it gets scared, the tail is released and flicks against the ground, flinging the springtail into the air in a flash.
How are millipedes different from centipedes?
Millipedes have two pairs of legs on almost every body segment and move slowly. They eat rotting leaves and curl up when disturbed.
What are pill bugs, and why do they curl into balls?
Pill bugs, or roly-polies, are not insects. They are crustaceans, related to crabs and shrimp.
Why do most animals that live in the soil see so poorly?
Under the ground there is little to no light, so eyes are not very useful. Many soil animals rely on touch, smell, and vibrations instead.
Do animals underground ever run out of air?
Usually not. Soil is full of air-filled gaps, and fresh air seeps in from above.
How does a groundhog survive winter in its burrow?
A groundhog eats a lot in the summer and fall to build up body fat. Then it goes into a deep burrow below the frost line and hibernates.
How can some frogs and toads dig themselves into the ground?
Spadefoot toads have hard, spade-shaped bumps on their hind feet. They wriggle backward and dig down into loose soil, sinking out of sight.
How can thousands of garter snakes share one winter den?
In Manitoba, red-sided garter snakes gather in deep cracks and caverns in limestone, where it stays above freezing all winter. Tens of thousands share these dens.
Why do so many kinds of bees nest in the ground?
About seven out of ten bee species dig their nests in the soil. Each female digs a tunnel and makes little rooms stocked with pollen for her babies.
How do animals find food in the dark ground?
In the dark soil, animals use smell, taste, touch, and vibrations. A worm follows chemicals from decaying leaves.
What do expert diggers' bodies have in common?
Animals that dig, such as badgers, aardvarks, and armadillos, have strong front limbs, big claws, powerful shoulders, and short, sturdy bodies. Many have small ears and eyes that are protected from dirt.
Why do many animals dig to escape heat and cold?
A few feet underground, the temperature stays much steadier than the air above. It's cooler in summer and warmer in winter.
What is a rabbit warren, and who lives in one?
A warren is a network of connected burrows with many entrances and chambers. European rabbits dig these tunnels and live in groups.
Why do prairie dogs build whole underground towns?
Prairie dogs live in groups called coteries within big towns of tunnels. Their burrows have several chambers for sleeping, nursing, and storing food, and they have lookout mounds at the entrances.
How do small insects survive the winter in the soil?
Many insects hide in the soil or leaf litter during winter, where it does not get as cold. Some, like bumblebee queens, dig into the ground and sleep.
How can a funnel and a lamp show you what lives in soil?
With a grown-up, put leaf litter in a funnel over a jar, with a screen to hold the litter. Shine a cool LED lamp above it.
How do soil animals share food and recycle nutrients?
In the soil, plants and animals that die become food. Bacteria and fungi break them down.
What jobs do roots do for a plant?
Roots have several jobs. They anchor the plant so wind and rain don't knock it over.
How do roots sense where gravity is pulling?
Roots grow downward because of gravity. In the tip of each root are tiny, heavy grains called statoliths that settle to the low side of the cells.
How do roots find water in the soil?
Roots can sense moisture. In drier soil, the tips bend toward the wetter side, which is called hydrotropism.
What are root hairs, and why do roots have so many?
Root hairs are tiny threadlike growths from a root's outer cells. They look like fuzz and are too small to see easily.
Why do roots grow around rocks instead of through them?
A root tip is soft and cannot push through solid stone. When it touches a rock, it feels the pressure and bends away, growing along the easiest path toward soil that has water and air.
What is the difference between a taproot and fibrous roots?
A taproot is a single thick main root that goes straight down, like a carrot or a dandelion. It can reach deep water and store food.
Which vegetables are really roots?
Carrots, beets, radishes, turnips, parsnips, and sweet potatoes are all storage roots. The plant stores sugar and starch in them to use later, such as when it flowers.
How far do tree roots spread?
Most tree roots are surprisingly shallow. The majority grow in the top foot or two of soil, where there is air, water, and nutrients.
Why do tree roots sometimes lift sidewalks and crack pipes?
Roots grow where air and water are available, often right under the pavement. As they get thicker, they push up on the slab above them and can lift it.
How do roots hold a hillside together?
Roots work like thousands of tiny ropes tying the soil together. They make a net that holds soil in place and helps rainwater soak in instead of rushing downhill.
How long could the roots of one small plant stretch?
In the 1930s, a scientist carefully washed the soil from one winter rye plant that had grown for four months. He measured over 380 miles of root, and billions of tiny root hairs.
What is a mycorrhiza, the partnership between roots and fungus?
A mycorrhiza is a team-up between a plant root and a fungus. The fungus's threads wrap around or enter the root and spread far into the soil, reaching water and minerals like phosphorus that roots cannot reach.
What are the thin threads of a fungus that spread underground?
The main body of most fungi is a mass of super-thin threads called mycelium. Each thread is much thinner than a hair, but together they form a web that spreads through soil, wood, and leaves.
Do trees really share food through underground fungus networks?
In some experiments, scientists, including Suzanne Simard in British Columbia, have found that carbon can move between trees through fungus threads. This idea is sometimes called the wood wide web.
How big can a single fungus grow?
One of the largest living things is a honey fungus in an Oregon forest. Its threads spread under the soil across about 2,400 acres, about as big as 1,800 football fields, and it has been growing for thousands of years.
Why do mushrooms sometimes grow in a circle called a fairy ring?
A fairy ring is a circle of mushrooms in a lawn or forest. It starts from one spot where a fungus grows underground from a single spore.
How do bean and pea roots get help from bacteria?
Plants need nitrogen to grow, and the air is full of it, but plants can't use it as a gas. Beans, peas, and clover have little lumps on their roots called nodules, filled with helpful bacteria.
Is a potato a root?
No, a potato is a tuber, a swollen underground stem that stores food for the plant. The little dents called eyes are buds that can sprout into new plants.
How do tulip and daffodil bulbs sleep through the cold?
A bulb, such as a tulip or daffodil, is a bundle of thick, fleshy leaves wrapped around a tiny flower bud. It stores food and water through the cold winter, safe below the ground.
Why is it so hard to dig up grass that spreads underground?
Many grasses have underground stems called rhizomes, which grow sideways below the surface. Along them, new shoots pop up.
How can a forest of trees be one single living thing?
Quaking aspens can grow new trunks from a single wide root system. A forest in Utah called Pando has about 47,000 trunks, all genetically identical and connected underground.
Why can a cut-down tree stump grow new shoots?
The roots still hold lots of stored food. If the stump and roots are alive, they can send up new shoots from buds near the base.
Why do prairie grasses have roots deeper than the plants are tall?
Prairie grasses like big bluestem can have roots reaching 10 feet or more underground, while the tops reach only 6 feet or so. Deep roots find water during dry spells, and they hold soil in the wind.
What is the rhizosphere, the busy zone around a root?
The rhizosphere is the narrow zone of soil, only a few millimeters thick, around each root. Roots leak sugars and other chemicals into it, which attract bacteria and fungi.
How can scientists watch roots grow underground?
Scientists use clear tubes, called minirhizotrons, put into the soil with tiny cameras inside. Some research stations have underground rooms with glass walls so that researchers can see roots through the window.
How does an excavator dig so powerfully?
An excavator is powered by liquid pushed through tubes. An engine runs a pump that squeezes oil, called hydraulic fluid, into cylinders.
How does a tunnel boring machine work?
A tunnel boring machine, or TBM, is a giant tube with a spinning cutting wheel at the front. The wheel's hard metal discs grind through soil and rock.
What holds up the roof while a tunnel is being dug?
Digging leaves an open space that could sag or collapse, so builders add support as they go. In hard rock, workers put long steel bolts into the ceiling and spray concrete on the walls.
How do workers get fresh air in a long tunnel?
Digging a tunnel makes dust and engine fumes, so builders keep fresh air moving all the time. Huge fans push clean air through big tubes or duct pipes to the working end.
How do builders dig a tunnel from both ends and meet in the middle?
Surveyors measure everything very carefully, using lasers and satellites to mark the tunnel's exact path. Crews can then start digging from two sides, aiming at the same point.
How long can a tunnel be?
The Gotthard Base Tunnel in Switzerland, a train tunnel, is about 35 miles long and goes under the Alps. The longest road tunnel, in Norway, is about 15 miles.
How do builders make tunnels under rivers and seas?
One method is to dig a trench in the river bottom and sink tunnel sections built on land. The sections float like boats and then are lowered and joined in the trench.
How did people dig tunnels before machines?
For thousands of years, people used hand tools like picks, hammers, and baskets. In dry lands, builders dug long, gently sloping tunnels called qanats to bring water from hills to farms and towns.
What is a mine?
A mine is a place where people dig up useful materials from the ground. In an underground mine, shafts and tunnels reach deep into rock.
What useful things come out of mines?
Mines give us iron for steel, copper for wires, aluminum from a rock called bauxite, nickel, gold, silver, and many more. They also give us salt, sand, gravel, and coal.
How do miners reach a deep ore deposit?
Miners dig a shaft, which is a vertical tunnel with a cage that works like an elevator, or a ramp that spirals downward for trucks. From the shaft, flat tunnels branch out to the rock that holds the ore.
How do mines stay safe for workers?
Underground mines have many safety measures. Fans bring in fresh air and sensors test for gas.
Why is there a giant salt mine under Lake Huron?
About 400 million years ago, a shallow sea in this region dried up, leaving thick layers of salt that were later covered by rock. At Goderich, Ontario, workers dig this salt from about 1,800 feet below the ground, under the lake.
Why do open-pit mines look like giant steps?
In an open-pit mine, workers dig down in wide steps, called benches. This shape keeps the walls from collapsing and gives trucks a spiral road to drive up and down.
Why are mining trucks so huge?
A mine moves millions of tons of rock, so it needs trucks that carry as much as possible in one trip. The biggest haul trucks carry about 400 tons, which is more than the weight of 70 elephants.
How do we get metal out of rock?
Ore is rock with a small amount of metal inside. First, the ore is crushed and ground into powder.
Why is Sudbury, Ontario, famous for mining?
About 1.85 billion years ago, a giant space rock hit what is now Sudbury and created a huge crater. The impact helped bring up rock rich in nickel and copper.
How deep is the deepest mine in the world?
The Mponeng gold mine in South Africa goes about 2.5 miles deep, which is about seven times taller than the CN Tower. The rock down there can be as hot as 140 degrees...
How do miners break up hard rock?
Miners drill rows of holes into the rock face. Trained blasting crews load them with carefully measured charges, clear everyone to a safe distance, and set them off.
Why do some cities build walkways underground?
In cold, snowy places, underground walkways let people travel between buildings and subway stations without going outside. Toronto's PATH has about 19 miles of connected tunnels, with shops and cafés.
How do builders dig a deep basement without the sides caving in?
Before digging, workers often drive strong steel beams or build concrete walls around the edge to hold back the soil. As they dig, they add braces across the hole.
Why do root cellars keep food fresh?
A root cellar is a room dug into the ground or a hillside. A few feet down, the temperature stays cool and steady all year, even when the weather above is freezing or hot.
Why do scientists build laboratories far underground?
Some experiments hunt for super-rare tiny particles from space, such as neutrinos. Cosmic rays from space would swamp the detectors on the surface.
What is the tall tower over a mine called?
The tower is called a headframe. It stands above the shaft and holds big wheels called sheaves.
How do pumps keep underground spaces dry?
Water seeps through the rock and soil into mines and tunnels. If it was not removed, the spaces would flood.
What is peat, and how does it turn into coal?
Peat is dark, spongy material made from partly rotted plants that pile up in wet bogs. Canada has huge peatlands.
Why do some oil wells stand out in the ocean?
Some oil lies in rock far below the sea floor. To reach it, workers build an offshore platform that sits on legs or floats and is held in place by strong anchors.
How do geologists find oil and gas hiding deep in the rock?
Geologists send sound waves into the ground and listen for the echoes. The waves bounce back differently from different layers of rock, like the echo from a canyon wall.
Why does oil stay trapped underground instead of floating to the top?
Oil is lighter than water, so it slowly rises through tiny holes in rock until it hits a layer it cannot pass through, called cap rock. The oil collects in the porous rock below it, like water in a sponge under a plastic lid.
How do fish that live in dark caves get by without eyes?
Some fish live in caves where no light ever shines. Over many generations, cave-dwelling populations of some species, such as the Mexican tetra, have lost their eyes and...
How does the Oak Ridges Moraine soak up rain like a giant sponge?
The Oak Ridges Moraine is a long ridge north of Toronto, built from sand and gravel that glaciers dropped about 12,000 years ago. Rain and melting snow soak through the...
How does a well bring water up from underground?
A well is a hole dug or drilled down below the water table. Water seeps in from the rock and soil.
What is an artesian well?
An artesian well happens when water is trapped between two layers of tight rock and tilted so that it is under pressure. When a well is drilled into it, the pressure...
Where does the water in a spring come from?
A spring is a place where groundwater flows out onto the surface. Rain and melting snow soak into the ground and move slowly downhill through cracks and spaces in the rock.
Why do some wells run dry?
Wells run dry when the water table drops below the bottom of the well. This can happen in a drought when little rain soaks in, or when more water is pumped out than gets refilled.
Can a forked stick really find water underground?
Some people walk around holding a forked stick, saying it dips down over water. This is called dowsing.
How do veins of metal form in rock?
Deep in Earth, hot water full of dissolved minerals moves through cracks in rock. As the water cools, the minerals come out of the water and build up on the walls of the crack, like the crust inside a kettle.
Why is gold found in streams?
Gold is very heavy, about 19 times heavier than water, and does not rust. When rock containing gold wears away, streams carry the pieces along.
Why are there huge piles of sand and gravel in some places?
Long ago, in the Ice Age, glaciers carried sand, gravel, and boulders over great distances. When the ice melted, it dropped them in piles and ridges.
How can solid rock be made of tiny shells?
Limestone is a rock made mostly from the shells and skeletons of sea creatures, such as corals, clams, and tiny plankton. They piled up on the seafloor for millions of years and were pressed into stone.
Where does the clay for bricks and pots come from?
Clay forms when rocks break down into very fine, flat particles. It is dug from the ground in clay pits.
Why is there a seed vault inside a frozen mountain?
The Svalbard Global Seed Vault is a safe storage place dug into a mountain in the Arctic. It keeps backup copies of seeds from crops from around the world, in case a farm or seed bank loses its seeds.
What is a time capsule?
A time capsule is a sealed container filled with items and buried or hidden to be opened in the future. People put in newspapers, photos, letters, and objects that show what life is like now.
Why do old things get buried deeper over time?
Over many years, dust, leaves, and soil build up on the ground. Earthworms also bring soil from below up to the surface in their castings.
What causes a landslide, and what are the warning signs?
A landslide happens when a slope of soil and rock slides downhill. Heavy rain, melting snow, or an earthquake can make the ground heavy and slippery.
Why do basements feel cool and damp?
A basement is surrounded by soil, which stays near the same cool temperature all year. The ground keeps the walls and floor cool.
How does loose sand become solid sandstone?
Sand that is buried deep under more layers gets squeezed by their weight. Water seeping through it carries dissolved minerals, such as silica or calcite, which fill the gaps between the grains.
How do quarries cut big blocks of stone?
A quarry is an open mine for stone. Workers use machines to drill lines of holes into a rock face and wire saws to cut big blocks free.
What else do we make from oil besides fuel?
Oil is a mixture, and it can be separated into different parts. Some become gasoline, diesel, and jet fuel.
How can people use the ground's treasures wisely?
Minerals, metals, and fuels in the ground take millions of years to form, so once they are gone, they are gone. People can use them wisely by saving energy, repairing and reusing things, and recycling metals and plastics.
How can a machine see underground without digging?
Ground-penetrating radar sends tiny pulses of radio waves into the ground and listens for echoes. When the waves hit something different, like a pipe, a buried wall, or a layer of rock, some bounce back.
How can a tiny camera peek down a narrow hole?
Scientists and well drillers lower a small waterproof camera on a cable into a borehole, which is a narrow hole drilled into the ground. A bright light on the camera...
How do scientists take a core sample of soil?
A soil corer is a hollow metal tube that is pushed or hammered into the ground. When it is pulled out, it holds a column of soil with all the layers in order.
How do archaeologists map a dig site?
Before digging, archaeologists mark off the site in a grid of squares using string and stakes. They dig each square a little at a time, record the depth, and take photos and notes of everything they find.
What happens to a fragile artifact after it comes out of the ground?
Once an artifact is dug up, air and light can start to damage it. Archaeologists brush off the soil gently, take photos, and note exactly where it was found.
Why must crews call before they dig?
Underground, there are pipes for water and gas, electric cables, and phone and internet lines, and hitting one by mistake can be dangerous. Before digging, workers call a free service.
What is humus, the dark crumbly part of soil?
Humus is the dark, crumbly material that forms when dead leaves, roots, and animal bodies break down for a long time. It soaks up water like a sponge, holds nutrients...
How can you tell what kind of soil you have just by squeezing it?
Moisten a handful of soil and squeeze. Sandy soil feels gritty and falls apart.
How can scientists tell if soil is thirsty or soaked?
Farmers and scientists push probes into the ground that measure how easily electricity moves through the soil. Wet soil carries more than dry soil, so the probe can tell the moisture level.
How can dogs find food that grows underground?
Truffles are fungi that grow underground near tree roots. They give off a strong smell that dogs can detect from far away.
How do scientists count the creatures living in the ground?
Scientists mark a square patch of soil, dig out the top layer, and gently sort through it by hand, counting worms and insects. Another method is to bury a small cup so its top is level with the ground.
Can scientists listen to life in the soil?
Yes. Researchers push tiny microphones into the ground and record rustling, scratching, and clicking sounds made by worms, beetles, ants, and other tiny animals.
How does carbon dating tell the age of buried wood or bones?
All living things take in a tiny amount of a special kind of carbon called carbon-14. When they die, they stop taking in more, and the carbon-14 they have slowly fades away at a steady rate.
What secrets can a drilled rock core hold?
A rock core is a long cylinder drilled out of deep rock. It shows layers like pages in a book.
Why do ghostly lines show up in dry grass or crops?
Buried stone walls or roads make the soil shallow above them, so plants dry out faster and turn yellow. Buried ditches hold more water, so plants above them grow greener and taller.
Why should you never go into an old well, mine, or tunnel?
Old underground places can be dangerous. The walls and roof may be weak and could fall, the air may not have enough oxygen, and there may be hidden drops or water.
Why can a deep hole in the sand cave in?
Dry or wet sand has no glue to hold the grains together, so the walls of a hole can slump. Sand is also heavy, and a collapse can trap a person.
Why can squished soil make it hard for plants to grow?
Heavy machines, feet, and tires press soil grains tightly together and squeeze out the air gaps. Roots can't push through hard soil, water runs off instead of soaking in, and soil animals have no room to move.
How can people stop soil from washing or blowing away?
Bare soil is easily carried away by rain and wind, which is called erosion. People protect it with plants and mulch.
How can we protect the water hidden in the ground?
Groundwater can be polluted by things that soak in from above. People protect it by never pouring paint, oil, or chemicals on the ground or down drains, by taking them to special collection sites, and by sealing old wells.
How do people heal land after a mine closes?
When a mine is finished, workers can fill holes, reshape the land, and add fresh soil. Then they plant grasses and trees.
What goes in an underground detective kit?
A good soil-detective kit has a magnifying glass, a small trowel (used with a grown-up's help), jars, a ruler, a notebook, and a thermometer. Add a bit of string for marking squares, plain paper, and a pencil.
Who works with the ground for a living?
Geologists study rocks, and hydrogeologists study groundwater. Soil scientists look at the ground and plants.
What puzzles about the ground are scientists still chasing?
Scientists still do not know how many kinds of living things live in soil, because most are tiny and unstudied. They are curious about how deep life reaches and how fungus networks really share things.
What is the big story of the world beneath our feet?
Under every step you take is a hidden world. There are layers of soil and rock, water seeping through, roots and fungi tied together, and countless tiny animals at work.
From Book 1: The Great Expedition (5)
Why Does Soil Matter? The Ground Is Full of Life!
Soil is more than dirt! It starts with broken rock crumbs mixed with air, water and tiny living things.
Why Do Caves Form? Rain, Rock and Slow Drips!
Caves are nature's underground architecture. Rain picks up carbon dioxide and becomes a weak carbonic acid, which seeps into the ground and slowly dissolves limestone along cracks until they become channels and passages.
Why Do Geysers Shoot? Nature's Underground Plumbing!
A geyser is nature's underground plumbing at work! Rain and snowmelt seep deep underground through cracks and porous rock, and the water gets trapped in chambers where it heats way past boiling.
How Do Clocks Keep Time? Ticks, Swings and Crystals!
A clock counts equal ticks, and that is how it keeps time. Old clocks used swinging pendulums to count, and many clocks now count tiny crystal vibrations.
Why Does Ice Float? Water's Roomy Crystals!
Most solids sink in their own liquid, but ice floats! When water freezes, it locks into a roomy crystal made of hexagons, and the extra empty space makes ice less dense so it floats.
From Book 3: The Built Expedition (2)
From Book 4: The Wild Expedition (3)
Why Do Roots Grow Down?
Roots grow down because they follow gravity. Down is where soil holds moisture, and where a plant can anchor itself.
Why Do Plants Drink Through Roots?
Roots are the plant's drinking straws. Tiny hairs pull water and minerals from the soil, so the leaves can cook food in the sun.
Why Do Worms Come Out After Rain?
Wet ground fills the worm's tunnels with water. Worms breathe through damp skin, and coming up is often easier than staying in a flooded tube.
From Book 5: The Time Expedition (9)
Dig Detectives! Why Use Brushes Instead of Shovels?
Fossils can be fragile, and a shovel could crack or crush them. Scientists use soft brushes and small tools to clear away dirt very gently.
Save Some for Later! Why Leave a Dig Site Half-Buried?
Sometimes scientists choose not to dig everything up at once. Leaving part of a site in the ground can protect it, and future scientists with new tools and new questions may learn even more.
Still Digging! Why Do We Find New Fossils Every Year?
There are so many places on Earth that have not been searched carefully yet. Wind, water, and weather can uncover fossils that were hidden for millions of years.
More Than Mud! Why Is Soil More Than Dirt?
Soil is a mix of tiny bits of rock, air, water, and rotted plant and animal material. It is also home to worms, insects, and lots of tiny living things.
Underground Worlds! Why Do Caves Form?
Rainwater picks up a little acid from the air and soil. Where the rock is limestone, that slightly acidic water can slowly dissolve the rock and carve out empty spaces underground.
Cave Artists! Why Did People Paint on Cave Walls?
People painted animals and shapes on cave walls thousands of years ago, and some of those paintings are still there today. Scientists are not sure exactly why they did it.
Hello From the Past! Why Are Handprints in Caves a Clue?
Some caves have handprints that were made thousands of years ago. A handprint shows that a real person was there, and it can give hints about who made it.
Back to the Dig! Why Return to the Same Site?
A site can hold more clues than scientists can uncover in one visit. Coming back lets them dig more, ask new questions, and use new tools.
Tiny Details! Why Use Microscopes on Old Dirt?
Old dirt can hold very small clues like tiny seeds, pollen, bits of bone, and pieces of charcoal. A microscope helps scientists see them.
From Book 6: The Ocean Expedition (1)
From Book 9: The Food Expedition (2)
Why Do Muffins Get Tunnels If You Mix Too Much?
Muffins get tunnels when you mix too much because overmixing builds extra gluten and big gas tunnels! Gentle mixing keeps the crumb tender and even.
Why Do Farms Use Soil So Carefully?
Farms use soil carefully because soil is a living factory that grows our food! It holds water, nutrients, and billions of tiny microbes that plants need.
From Book 11: The Animal Expedition (10)
Why Do Worms Not Have Fur, Feathers, or Scales?
Worms have no fur, feathers, or scales because they breathe through their skin! An earthworm's skin has to stay thin and moist so air can pass through.
Why Do Naked Mole Rats Share Tunnels?
Naked mole rats share tunnels because they live in big underground colonies, a bit like ants! One queen has the babies, and everyone else has a job.
Why Do Antlions Dig Tiny Pits in the Sand?
Antlions dig tiny pits in the sand because the pits are sneaky traps for ants! The sand is so loose that the sides slide away.
Why Do Burrowing Owls Live Underground?
Burrowing owls live underground because it keeps them cool, warm, and safe! They live on grassy prairies where there are very few trees.
Why Do Polar Bears Dig Snow Dens?
Polar bears dig snow dens because snow is a great blanket for newborn cubs! Mother bears dig the dens in big snowdrifts, and outside it may be freezing while inside stays much warmer.
Why Do Foxes Dig Dens with More than One Door?
Foxes dig dens with more than one door so they always have a way out! A backup exit keeps the fox and her kits safe.
Why Do Moles Dig Such Long Tunnels?
Moles dig long tunnels because the tunnels are both their home and their hunting ground! Worms and insects wander into the tunnels.
Why Do Bats Roost in Caves?
Bats roost in caves because caves are dark, cool, and steady all year! That makes a safe place to sleep during the day.
Why Do Ghost Crabs Dig Burrows on the Beach?
Ghost crabs dig burrows on the beach to stay cool, damp, and hidden! The hot sun can dry a crab out.
Why Do Trapdoor Spiders Live in Burrows with Lids?
Trapdoor spiders live in burrows with lids because the lid hides them and keeps the burrow moist! The spider waits in the shadows.
From Book 12: The Volcano Expedition (6)
Why Do People Study Ash Layers in Soil?
Every eruption can spread a thin layer of ash over a huge area. The ash gets buried in soil, lakes, and ice.
Why Does Shaking Feel Worse on Soft Soil?
Soft, loose ground shakes more, and for longer, than hard rock. Think of a bowl of jelly wobbling on a table.
Why Are Some Earthquakes Deep Underground and Others Close to the Surface?
Some earthquakes start deep underground, and others start close to the surface. The place where the rock first slips is called the focus.
Why Do Continents Have Deep Roots?
Continents have deep roots, just like an iceberg floating in the sea! Thick, light crust floats on the heavier rock of the mantle.
Why Do Volcanoes Create Fertile Soil Later?
Volcanoes can create super fertile soil, but only later, after many years! When lava and ash slowly break down, they turn into rich dirt.
How Do Diamonds Come Up From Deep Inside Earth?
Diamonds form deep underground, over 150 kilometers down, where heat and pressure squeeze carbon into crystals. A special, speedy magma called kimberlite blasts upward...
From Book 13: The Light Expedition (2)
Why Is Table Salt White When One Crystal Is Clear?
A single grain of salt is a tiny clear crystal. But a pile of grains has lots of surfaces that bounce light in every direction.
Why Are Tunnels Extra Bright Near the Entrance?
Tunnels are extra bright near the entrance so your eyes can adjust! Going from bright sunshine into a dim tunnel feels like going into darkness.
From Book 14: The Sound Expedition (1)
From Book 15: The Robot Expedition (1)
From Book 16: The Desert Expedition (10)
Why Is It So Much Cooler a Few Feet Underground in a Desert?
A few feet underground, a desert can be much cooler than the surface! Soil and sand are poor conductors of heat, so the Sun's warmth takes a long time to move downward.
Why Do Some Desert Shrubs Send Their Roots Down 50 Feet or More?
Some desert shrubs send roots down 50 feet or more to reach water far below! Deep underground, the soil stays damp long after the surface dries out.
Why Do Some Desert Plants Spread Their Roots Wide but Shallow?
Some desert plants spread their roots wide but shallow to catch rain fast! A desert rain soaks only the top few inches of soil, and it dries quickly.
How Do Mesquite Trees Make Poor Desert Soil Richer?
Mesquite trees make poor desert soil richer by teaming up with tiny helpers in their roots! Those helpers are bacteria that make plant food from the air.
Why Does the Bilby Dig So Many Burrows?
A bilby digs many burrows to stay cool by day and safe from hunters! A bilby is a rabbit-sized animal from Australia.
Why Are Desert Tortoise Burrows Like Apartment Buildings?
Desert tortoise burrows are like apartment buildings, because many animals move in! The burrow stays cool in summer and warm in winter, which is just right for a desert.
How Does Water Hide Underground in a Desert?
Water hides underground in a desert by soaking into sand and rock, like water in a wet sponge! This hidden water is called groundwater.
Why Doesn’t Desert Soil Soak Up Rain Well?
Desert soil does not soak up rain well because it is often baked hard or sealed by a thin crust! There are also few plant roots to open channels for water.
How Do Scientists Find Water That Is Hidden Underground?
Scientists find hidden water by combining many clues, like a team of detectives! They study rocks, drill test wells, and use tools that peek underground.
What Is the Crust on Desert Soil, and Why Shouldn’t You Step on It?
The crust on desert soil is a living skin made of tiny living things! Because it is alive and fragile, you should never step on it.
From Book 17: The Rainforest Expedition (6)
Why Is Rainforest Soil Poor Even Though Plants Grow So Thickly?
Rainforest soils are old, and heavy rain keeps washing the nutrients down and away. Most nutrients are locked inside living plants and fresh leaf litter.
How Can Giant Trees Grow in Poor Soil?
Most rainforest trees have shallow, wide roots that spread through the top few inches of soil. That is where fallen leaves rot and release nutrients.
What Is Humus, and Why Is the Layer of Good Soil So Thin in a Rainforest?
Humus is dark, crumbly material made from rotted plants and animals. It stores nutrients and water for plant roots.
Why Do Some Rainforest Trees Have Giant Flared Roots Like Walls?
Buttress roots are giant, wall-like ridges that help tall trees stand up! Rainforest soil is thin, so tall trees need extra support.
What Lives in a Handful of Rainforest Soil?
A handful of rainforest soil is like a tiny hidden city! It is packed with living things, most too small to see without a microscope.
Why Does Soil Wash Away When Rainforest Trees Are Cut?
Soil washes away when trees are cut because trees protect the ground! Leaves slow down the rain, and roots hold the soil in place.
From Book 18: The Polar Expedition (2)
From Book 19: The River Expedition (2)
Can a River Disappear Underground?
Yes, a river can disappear underground! In places with limestone rock, water can dissolve the rock and form holes and caves.
Why Does a River Dig Deeper on the Outside of a Bend?
A river digs deeper on the outside of a bend because the water there flows faster! The faster water scrapes and wears away the bank.
From Book 20: The Mountain Expedition (4)
Do Mountains Have Roots?
Yes, in a way, mountains have roots! Earth's crust floats on the softer rock below it, like an iceberg in water.
Why Is Mountain Soil So Thin?
Mountain soil is thin because it is hard to build and easy to lose! On steep slopes, rain and melting snow wash soil downhill.
How Does a Ptarmigan Survive in a Snow Burrow?
A ptarmigan survives cold nights by tucking into a snow burrow! It sounds chilly, but snow is a great blanket.
Why Do Snow Scientists Dig Pits in the Snow?
Snow scientists dig pits so they can see inside the snowpack! A pit works like a window, because the wall shows every layer in a cross-section.
