Will sat at his desk, staring at a picture in his textbook.
"How can huge holes just appear underground? I don't understand," he thought.
He noticed it was already ten o'clock. "I'm so tired I can't even think anymore!
I'll finish this report tomorrow morning," he decided and climbed into bed. Within minutes he was asleep.
Will woke up shivering. He opened his eyes but saw nothing—just blackness!
It was eerily quiet, except for a constant dripping.
Then QT's screen lit up, and Will realized he wasn't in his room anymore.
"You are in Mammoth Cave, in Kentucky," QT said.
"This is the world's longest cave system. People have explored almost six hundred kilometers of tunnels and caverns so far!"
"I wouldn't want to get lost down here," Will murmured. Then, trying to be brave, he asked, "How are caves formed, anyway?"
"Well, there are many types of caves. I'll show you," QT said and projected an image of a cliff on the cave wall.
"Sea caves are made by waves beating on rock for a very long time, hundreds and thousands of years.
The wind can also carve out caves. You often see these in deserts. There are caves in lava tubes too.
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.
Sometimes caves are even formed in glaciers when water flows through the cracks, making them into big holes."
Will looked at the fantastic formations on the ceiling. "This cave doesn't look like any of those."
"We are in a solution cave, the most common type.
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."
Will was amazed. "Water made this cave?"
"Well, it had a little help. While water is traveling through soil, it becomes slightly acidic.
Limestone dissolves easily in acid—sort of like sugar in hot tea—
so when water flows through cracks in limestone for thousands of years, it forms these enormous caverns.
When the water table drops, these caves are left empty, or rather, full of air."
"So that's what makes the holes, but what makes these weird rocks?" Will asked.
"I'm getting to that. When the acidic water dissolves limestone, it absorbs the mineral from the stone.
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.
Every time a water droplet falls, a little of that mineral stays on the ceiling.
Over time the mineral builds up to form stalactites, these cones hanging from the ceiling.
When the drop hits the floor, a little of the mineral is deposited there too, which builds stalagmites, the cones rising from the floor."
"Stalactites and stalagmites . . . I'll never remember which is which!" Will interrupted.
"Here's a trick to help you keep them straight: stalactites hold tight to the ceiling, and stalagmites might reach the ceiling," QT offered.
"Anyway, they sometimes grow so long that they meet in the middle, forming a column."
"Cool," Will remarked. As he continued down the tunnel, he saw another huge formation. "What's that?"
"It's flowstone. It's really common in caves," QT explained.
"And those thin tubes hanging from the ceiling are called soda straws, for obvious reasons."
Will reached for one, and QT beeped in alarm. "Don't touch the formations! They're very fragile!"
After Will pulled his hand back, QT continued. "They can break, and the oil on our hands interferes with their growth."
"Oh. Okay." Will didn't try to touch anything else as he walked along.
Finally QT said, "Well, we've reached the deepest part of this cave system.
It's also the youngest part. We're over one hundred meters below the surface.
You know you've reached the bottom when you see lots of water. There are many underground rivers and lakes."
"And there's something swimming in the water!" Will exclaimed.
"Believe it or not, certain species spend their entire lives in caves.
Caves are unique because the conditions inside them stay the same all year.
It's always dark, humid, and the temperature doesn't change much.
These species have adapted to help them survive here.
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."
"It's completely white too!"
"Most cave species are very pale or white.
Since no animal can see without light, there's no need to spend valuable energy developing color."
Will walked up a steep tunnel branching away from the river.
After a few minutes, he leaned against the wall to rest.
"Ahhh!" Will screamed and jumped away. "Something touched my hand!"
He looked closely and saw a little bug.
"That's just a cave cricket," QT explained. "It's harmless.
Its long limbs and antennae are another adaptation.
They help the cricket find food and avoid predators, since it can't see."
Will continued to climb, and soon a glimmer of light appeared ahead.
He reached the top of the tunnel and crawled inside a huge cave.
"Yikes!" Will froze when he spotted dozens of bats flying near the ceiling.
"Don't be afraid, Will," QT said. "Bats aren't dangerous.
They're quite useful; a single bat can eat three thousand bugs a night!
Because they hunt in darkness—in caves and outside at night—they rely on sound instead of sight to find insects."
A bat flew so close to Will's face that he felt the breeze from its wings.
He closed his eyes and took a deep breath before moving again.
When he opened his eyes, he saw that he was back in his room. A breeze drifted in through the window.
"Whew. I'm glad I don't live in a cave," he said, "but maybe I'll ask Mom if we can visit one."