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Are There Underground Cities?

Courtney Bonnell
Rooms used by early Christians at the Kaymakli Underground City in the region of Cappadocia, Turkey.

In this episode, we answer a question from 5-year-old Wyatt in Los Angeles and learn about ancient underground cities in Turkey, the subterranean passageways of Montreal and the dug-out houses of Coober Pedy, Australia. Also in this episode: Why is it so warm underground?


"Why is underground so warm?" - Camden, Missoula, MT

It's not that it's necessarily warm underground, it's that the temperature is constant, meaning it doesn't change much.

That's because the outer layers of the earth, known as the crust, have stored thermal energy that helps keep the temperatures constant. And those inner layers are sheltered from the weather and air temperature fluctuations, their temperature doesn't change. Instead of being cold though, the earth has heat. Think of the earth as like an onion with lots of different layers. We'll start at the center. The inner core is solid iron, surrounded by an outer core: molten rock called magma. The next layer is called the mantle and it's made of magma and rock. Finally, there's the crust, which is 35 miles thick in some places and 3 to 5 miles thick under the ocean.

All of those layers of rock are heated by that magma deep in the core. The rocks get hotter the deeper you go. But the crust closet to the surface stays about 50 to 60 degrees all year round. In fact, some homes and businesses use that constant temperature to provide heating and cooling without being underground. They use technology called geo-thermal heat pumps to heat and cool buildings.

Click listen to hear the entire episode.

Virtual tour of the Umoona Opal Mine & Museum

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Melody is the Contributing Editor for But Why: A Podcast For Curious Kids and the co-author of two But Why books with Jane Lindholm.
Jane Lindholm is the host, executive producer and creator of But Why: A Podcast For Curious Kids. In addition to her work on our international kids show, she produces special projects for Vermont Public. Until March 2021, she was host and editor of the award-winning Vermont Public program Vermont Edition.
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