MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Illustration of the Earth, with the continents in their present form, but with the planet completely iced over. The snowball Earth hypothesis suggests that, hundreds of millions of years ago, the Earth may have frozen solid like this as a result of severe climate change. This illustration imagines what the planet might look like were this to happen now. In this view, we can make out the American continents.
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