Planet Earth: An Illustrated History

Cavern
Ice caverns like this one in Iceland may resemble caverns of rock, but they are, fact, quite different. While stone caves may take eons to form, cathedral-sized galleries of ice can be carved within glaciers by meltwater in years or even months. And unlike rooms gouged from solid rock, glacier caves can disappear even faster than they were formed. Stone caverns rarely collapse, but ice caves are always vulnerable to cave-in. Veteran glacier spelunkers, or glaciospeleologists, as they call themselves, tell of striking walls inside an ice cave with a hammer to anchor a rope, only to watch a crack hundreds of feet long appear with lightning-fast speed.

• The Book, "TIME Planet Earth: An Illustrated History"