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Jessi: Hi, welcome back to the fort! Did you know that the fort is a special secret underground hideout? It's a cave! A cave is just an underground space that’s big enough for a person to get into... or a robot like Squeaks! Caves can be really small, narrow passages, or they can be a lot bigger -- like our fort -- with a whole network of tunnels that connect different rooms, or chambers -- Some caves can be found on the sides of cliffs. Others are in forests. And some are even underwater. But all of those kinds of caves have one major thing in common: They’re made of rock.

Today, we’re going to talk about a different kind of super-cool - or should I say super cold? Cave: ice caves! Ice caves are a lot like other caves - they’re often underground and they’re large enough for people to get into. But instead of rock, they’re made of ice! There are two kinds of caves that are made of ice. The first kind is called a glacier cave.

Glacier caves are made completely of ice. They form in glaciers, which are huge chunks of ice that move like slow, frozen rivers. The most common way for a glacier cave to form is when melted water runs through, or under, the glacier. Because the water is warmer than the ice that it’s running over, it slowly melts the ice around it it, creating a tunnel, which eventually forms a cave.

The second kind of cave made from ice is called... an ice cave. To be called a true ice cave, a cave doesn’t need to be made completely of ice, like glacier caves are. But it does have to have ice in it year round, and at least part of the cave has to have a temperature below 0 degrees Celsius. Or 32 degrees Fahrenheit. Ohh! Brrrrr! Unlike glacier caves, Ice caves can form just about anywhere. They form when cold air enters a space where it can’t escape. These spaces then trap the cold air inside, so any water that trickles in, quickly freezes and stays frozen, no matter what time of year it is.

Ice caves can be pretty small - small enough to fit just one person, or they can be really big. The world’s largest ice cave is in Austria, Eisriesenwelt. Which is German for "World of the Ice Giants.” The cave is almost 50 kilometers long, and parts of it are open to visitors. So, if you’re ever lucky enough to go inside, you’ll be able to see all kinds of amazing ice formations!

And even though ice caves like this one may look like they’re empty, there are living creatures that make their home in ice caves and glacier caves. But there aren’t many, because very few living things can survive their freezing temperatures. And in case you’re wondering, you may not find any “ice giants” living in ice caves. But I can tell you that things that live in these spaces are often... really tiny! As tiny as... a water bear! Also known as a tardigrade.

Tardigrades are super-small animals known as micro-animals that you can only see under a microscope! Tardigrades can survive in really harsh environments that other animals couldn’t, like at the very bottom of the ocean, and, you guessed it - in glacier caves and ice caves! So if you ever find yourself in an ice cave, you might feel like you’re completely alone in the quiet, cold space. But you’re actually, probably surrounded by lots of tiny little animals.

Oh Man! Looking at all these pictures of ice caves has made me chilly. I'm glad our fort is a nice, warm, regular rock cave! Thanks for learning about these cool caves with us! And remember, if you have a question about anything you’d like to learn more about, just let us know by getting help from a grown up, and leaving a comment below. Or send us an email at kids@thescishow.com, and we'll see you next time!

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