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Listen to Book 24: The Underground Expedition

All 150 answers from Book 24, read aloud. Press play and keep listening, or tap a question to jump to it. Each title links to its page with the video and the full answer.

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1.What would you find if you dug a hole straight down?
2.What makes topsoil dark and crumbly?
3.What is subsoil, the lighter layer below the topsoil?
4.What is bedrock?
5.What is the deepest hole people have ever drilled?
6.Why does it get hotter the deeper you go underground?
7.What is the water table?
8.Why does mud get so sticky and slippery?
9.What is the frost line, and why are pipes buried below it?
10.What is a pingo, a hill with an ice heart?
11.How can solid-looking ground hold both air and water?
12.Why is soil different colors in different places?
13.Why does frozen ground push rocks up to the surface?
14.Why does water sink through sand but sit on top of clay?
15.Where does rainwater go when it soaks into the ground?
16.Why does the ground sometimes sink to make a sinkhole?
17.Why is there always leftover dirt when you fill a hole back in?
18.Why can you see stripes in a road cut or riverbank?
19.How long does it take to make an inch of topsoil?
20.What happens to things we bury in the ground?
21.How can a jar of muddy water show the layers in soil?
22.How deep does life go underground?
23.What is leaf litter, and what happens to it?
24.Why does the ground smell so fresh after rain?
25.Why is soil deep in some places and thin in others?
26.How many living things can fit in a spoonful of soil?
27.What is inside an ant nest?
28.How do ants get through the winter underground?
29.Why do some cicadas spend 13 or 17 years underground?
30.What are grubs, and what do they become?
31.How does a worm push through hard soil without legs?
32.How can a worm sense the world without eyes?
33.What are nematodes, and where do they live?
34.How do springtails jump when they have no wings?
35.How are millipedes different from centipedes?
36.What are pill bugs, and why do they curl into balls?
37.Why do most animals that live in the soil see so poorly?
38.Do animals underground ever run out of air?
39.How does a groundhog survive winter in its burrow?
40.How can some frogs and toads dig themselves into the ground?
41.How can thousands of garter snakes share one winter den?
42.Why do so many kinds of bees nest in the ground?
43.How do animals find food in the dark ground?
44.What do expert diggers' bodies have in common?
45.Why do many animals dig to escape heat and cold?
46.What is a rabbit warren, and who lives in one?
47.Why do prairie dogs build whole underground towns?
48.How do small insects survive the winter in the soil?
49.How can a funnel and a lamp show you what lives in soil?
50.How do soil animals share food and recycle nutrients?
51.What jobs do roots do for a plant?
52.How do roots sense where gravity is pulling?
53.How do roots find water in the soil?
54.What are root hairs, and why do roots have so many?
55.Why do roots grow around rocks instead of through them?
56.What is the difference between a taproot and fibrous roots?
57.Which vegetables are really roots?
58.How far do tree roots spread?
59.Why do tree roots sometimes lift sidewalks and crack pipes?
60.How do roots hold a hillside together?
61.How long could the roots of one small plant stretch?
62.What is a mycorrhiza, the partnership between roots and fungus?
63.What are the thin threads of a fungus that spread underground?
64.Do trees really share food through underground fungus networks?
65.How big can a single fungus grow?
66.Why do mushrooms sometimes grow in a circle called a fairy ring?
67.How do bean and pea roots get help from bacteria?
68.Is a potato a root?
69.How do tulip and daffodil bulbs sleep through the cold?
70.Why is it so hard to dig up grass that spreads underground?
71.How can a forest of trees be one single living thing?
72.Why can a cut-down tree stump grow new shoots?
73.Why do prairie grasses have roots deeper than the plants are tall?
74.What is the rhizosphere, the busy zone around a root?
75.How can scientists watch roots grow underground?
76.How does an excavator dig so powerfully?
77.How does a tunnel boring machine work?
78.What holds up the roof while a tunnel is being dug?
79.How do workers get fresh air in a long tunnel?
80.How do builders dig a tunnel from both ends and meet in the middle?
81.How long can a tunnel be?
82.How do builders make tunnels under rivers and seas?
83.How did people dig tunnels before machines?
84.What is a mine?
85.What useful things come out of mines?
86.How do miners reach a deep ore deposit?
87.How do mines stay safe for workers?
88.Why is there a giant salt mine under Lake Huron?
89.Why do open-pit mines look like giant steps?
90.Why are mining trucks so huge?
91.How do we get metal out of rock?
92.Why is Sudbury, Ontario, famous for mining?
93.How deep is the deepest mine in the world?
94.How do miners break up hard rock?
95.Why do some cities build walkways underground?
96.How do builders dig a deep basement without the sides caving in?
97.Why do root cellars keep food fresh?
98.Why do scientists build laboratories far underground?
99.What is the tall tower over a mine called?
100.How do pumps keep underground spaces dry?
101.What is peat, and how does it turn into coal?
102.Why do some oil wells stand out in the ocean?
103.How do geologists find oil and gas hiding deep in the rock?
104.Why does oil stay trapped underground instead of floating to the top?
105.How do fish that live in dark caves get by without eyes?
106.How does the Oak Ridges Moraine soak up rain like a giant sponge?
107.How does a well bring water up from underground?
108.What is an artesian well?
109.Where does the water in a spring come from?
110.Why do some wells run dry?
111.Can a forked stick really find water underground?
112.How do veins of metal form in rock?
113.Why is gold found in streams?
114.Why are there huge piles of sand and gravel in some places?
115.How can solid rock be made of tiny shells?
116.Where does the clay for bricks and pots come from?
117.Why is there a seed vault inside a frozen mountain?
118.What is a time capsule?
119.Why do old things get buried deeper over time?
120.What causes a landslide, and what are the warning signs?
121.Why do basements feel cool and damp?
122.How does loose sand become solid sandstone?
123.How do quarries cut big blocks of stone?
124.What else do we make from oil besides fuel?
125.How can people use the ground's treasures wisely?
126.How can a machine see underground without digging?
127.How can a tiny camera peek down a narrow hole?
128.How do scientists take a core sample of soil?
129.How do archaeologists map a dig site?
130.What happens to a fragile artifact after it comes out of the ground?
131.Why must crews call before they dig?
132.What is humus, the dark crumbly part of soil?
133.How can you tell what kind of soil you have just by squeezing it?
134.How can scientists tell if soil is thirsty or soaked?
135.How can dogs find food that grows underground?
136.How do scientists count the creatures living in the ground?
137.Can scientists listen to life in the soil?
138.How does carbon dating tell the age of buried wood or bones?
139.What secrets can a drilled rock core hold?
140.Why do ghostly lines show up in dry grass or crops?
141.Why should you never go into an old well, mine, or tunnel?
142.Why can a deep hole in the sand cave in?
143.Why can squished soil make it hard for plants to grow?
144.How can people stop soil from washing or blowing away?
145.How can we protect the water hidden in the ground?
146.How do people heal land after a mine closes?
147.What goes in an underground detective kit?
148.Who works with the ground for a living?
149.What puzzles about the ground are scientists still chasing?
150.What is the big story of the world beneath our feet?

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