The Earth is missing some of its crust, and now scientists have a new lead on what’s to blame: A lot of glaciers.

Nearly 720 million years ago, Earth was cloaked in global ice, an era known as Snowball Earth. The grinding of these worldwide ice sheets may have bulldozed between 1.8 and 3 miles (3 and 5 kilometers) of crust into the oceans, researchers reported Dec. 31. There, plate tectonics crunched it back into Earth’s hot middle layer, the mantle, recycling it into new rock.

Great Unconformity. Seen around the world, this unconformity refers to a layer where sedimentary rocks have been deposited right on top of the oldest basement rock of the crust. Bizarrely, hundreds of millions of years of sedimentary layers are missing between this igneous or metamorphic basement and the oldest preserved sedimentary rocks. In the Grand Canyon, for example, a mind-boggling 1.2 billion years of rock are simply missing.

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