1
00:00:06,200 --> 00:00:10,478
Will sat at his desk, staring at a picture in his textbook.
2
00:00:10,479 --> 00:00:17,375
"How can huge holes just appear underground? I don't understand," he thought.
3
00:00:17,376 --> 00:00:23,297
He noticed it was already ten o'clock. "I'm so tired I can't even think anymore!
4
00:00:23,298 --> 00:00:31,359
I'll finish this report tomorrow morning," he decided and climbed into bed. Within minutes he was asleep.
5
00:00:31,360 --> 00:00:36,274
Will woke up shivering. He opened his eyes but saw nothing—just blackness!
6
00:00:36,275 --> 00:00:40,656
It was eerily quiet, except for a constant dripping.
7
00:00:40,657 --> 00:00:45,596
Then QT's screen lit up, and Will realized he wasn't in his room anymore.
8
00:00:45,597 --> 00:00:50,754
"You are in Mammoth Cave, in Kentucky," QT said.
9
00:00:50,755 --> 00:01:00,729
"This is the world's longest cave system. People have explored almost six hundred kilometers of tunnels and caverns so far!"
10
00:01:00,730 --> 00:01:09,440
"I wouldn't want to get lost down here," Will murmured. Then, trying to be brave, he asked, "How are caves formed, anyway?"
11
00:01:09,441 --> 00:01:18,182
"Well, there are many types of caves. I'll show you," QT said and projected an image of a cliff on the cave wall.
12
00:01:18,183 --> 00:01:26,512
"Sea caves are made by waves beating on rock for a very long time, hundreds and thousands of years.
13
00:01:26,513 --> 00:01:35,239
The wind can also carve out caves. You often see these in deserts. There are caves in lava tubes too.
14
00:01:35,240 --> 00:01:46,671
When lava is flowing from a volcano, the top gets cool first and hardens, but the hot lava inside flows right out, leaving a cave behind.
15
00:01:46,672 --> 00:01:56,381
Sometimes caves are even formed in glaciers when water flows through the cracks, making them into big holes."
16
00:01:56,382 --> 00:02:02,217
Will looked at the fantastic formations on the ceiling. "This cave doesn't look like any of those."
17
00:02:02,218 --> 00:02:06,966
"We are in a solution cave, the most common type.
18
00:02:06,967 --> 00:02:17,921
This kind of cave is formed when water trickles through a layer of limestone in the ground to the water table, the level where our groundwater starts."
19
00:02:17,922 --> 00:02:22,075
Will was amazed. "Water made this cave?"
20
00:02:22,076 --> 00:02:30,118
"Well, it had a little help. While water is traveling through soil, it becomes slightly acidic.
21
00:02:30,119 --> 00:02:35,397
Limestone dissolves easily in acid—sort of like sugar in hot tea—
22
00:02:35,398 --> 00:02:42,798
so when water flows through cracks in limestone for thousands of years, it forms these enormous caverns.
23
00:02:42,799 --> 00:02:49,876
When the water table drops, these caves are left empty, or rather, full of air."
24
00:02:49,877 --> 00:02:55,281
"So that's what makes the holes, but what makes these weird rocks?" Will asked.
25
00:02:55,282 --> 00:03:03,199
"I'm getting to that. When the acidic water dissolves limestone, it absorbs the mineral from the stone.
26
00:03:03,200 --> 00:03:14,201
When the water drips from the ceiling of a cavern, it encounters air again and the opposite happens; the mineral dissolved in the water comes out.
27
00:03:14,202 --> 00:03:20,388
Every time a water droplet falls, a little of that mineral stays on the ceiling.
28
00:03:20,389 --> 00:03:27,914
Over time the mineral builds up to form stalactites, these cones hanging from the ceiling.
29
00:03:27,915 --> 00:03:38,792
When the drop hits the floor, a little of the mineral is deposited there too, which builds stalagmites, the cones rising from the floor."
30
00:03:38,793 --> 00:03:45,882
"Stalactites and stalagmites . . . I'll never remember which is which!" Will interrupted.
31
00:03:45,883 --> 00:03:57,206
"Here's a trick to help you keep them straight: stalactites hold tight to the ceiling, and stalagmites might reach the ceiling," QT offered.
32
00:03:57,207 --> 00:04:03,435
"Anyway, they sometimes grow so long that they meet in the middle, forming a column."
33
00:04:03,436 --> 00:04:11,073
"Cool," Will remarked. As he continued down the tunnel, he saw another huge formation. "What's that?"
34
00:04:11,074 --> 00:04:16,972
"It's flowstone. It's really common in caves," QT explained.
35
00:04:16,973 --> 00:04:22,769
"And those thin tubes hanging from the ceiling are called soda straws, for obvious reasons."
36
00:04:22,770 --> 00:04:29,863
Will reached for one, and QT beeped in alarm. "Don't touch the formations! They're very fragile!"
37
00:04:29,864 --> 00:04:38,188
After Will pulled his hand back, QT continued. "They can break, and the oil on our hands interferes with their growth."
38
00:04:38,189 --> 00:04:43,305
"Oh. Okay." Will didn't try to touch anything else as he walked along.
39
00:04:43,306 --> 00:04:49,233
Finally QT said, "Well, we've reached the deepest part of this cave system.
40
00:04:49,234 --> 00:04:55,097
It's also the youngest part. We're over one hundred meters below the surface.
41
00:04:55,098 --> 00:05:01,752
You know you've reached the bottom when you see lots of water. There are many underground rivers and lakes."
42
00:05:01,753 --> 00:05:05,967
"And there's something swimming in the water!" Will exclaimed.
43
00:05:05,968 --> 00:05:11,465
"Believe it or not, certain species spend their entire lives in caves.
44
00:05:11,466 --> 00:05:16,776
Caves are unique because the conditions inside them stay the same all year.
45
00:05:16,777 --> 00:05:21,601
It's always dark, humid, and the temperature doesn't change much.
46
00:05:21,602 --> 00:05:25,456
These species have adapted to help them survive here.
47
00:05:25,457 --> 00:05:36,308
Like many other cave creatures, that species of fish has lived in complete darkness so long that it's gone blind. But it's very sensitive to movement."
48
00:05:36,309 --> 00:05:38,800
"It's completely white too!"
49
00:05:38,801 --> 00:05:42,017
"Most cave species are very pale or white.
50
00:05:42,018 --> 00:05:49,106
Since no animal can see without light, there's no need to spend valuable energy developing color."
51
00:05:49,107 --> 00:05:52,344
Will walked up a steep tunnel branching away from the river.
52
00:05:52,345 --> 00:05:56,620
After a few minutes, he leaned against the wall to rest.
53
00:05:56,621 --> 00:06:01,868
"Ahhh!" Will screamed and jumped away. "Something touched my hand!"
54
00:06:01,869 --> 00:06:05,046
He looked closely and saw a little bug.
55
00:06:05,047 --> 00:06:10,610
"That's just a cave cricket," QT explained. "It's harmless.
56
00:06:10,611 --> 00:06:13,899
Its long limbs and antennae are another adaptation.
57
00:06:13,900 --> 00:06:18,702
They help the cricket find food and avoid predators, since it can't see."
58
00:06:18,703 --> 00:06:22,648
Will continued to climb, and soon a glimmer of light appeared ahead.
59
00:06:22,649 --> 00:06:26,942
He reached the top of the tunnel and crawled inside a huge cave.
60
00:06:26,943 --> 00:06:32,183
"Yikes!" Will froze when he spotted dozens of bats flying near the ceiling.
61
00:06:32,184 --> 00:06:36,641
"Don't be afraid, Will," QT said. "Bats aren't dangerous.
62
00:06:36,642 --> 00:06:42,337
They're quite useful; a single bat can eat three thousand bugs a night!
63
00:06:42,338 --> 00:06:53,379
Because they hunt in darkness—in caves and outside at night—they rely on sound instead of sight to find insects."
64
00:06:53,380 --> 00:06:57,802
A bat flew so close to Will's face that he felt the breeze from its wings.
65
00:06:57,803 --> 00:07:02,063
He closed his eyes and took a deep breath before moving again.
66
00:07:02,064 --> 00:07:08,503
When he opened his eyes, he saw that he was back in his room. A breeze drifted in through the window.
67
00:07:08,504 --> 00:07:17,524
"Whew. I'm glad I don't live in a cave," he said, "but maybe I'll ask Mom if we can visit one."