A stone the size of your fist drops through seawater and punches into soft mud. The layers bend beneath it. Floating ice carried the stone out, then let it fall.
That dropstone is a clue to ice above water. Magnetic grains in nearby rock add another clue: some old glacial rocks formed near the equator. Ice had reached far beyond the poles.
The ocean's surface froze over across vast areas. Liquid water remained underneath. Think of a lid of ice above a sea that still moved, with hot vents on its floor.
As ice spread, it bounced more sunlight back into space and helped cool the surface. Fresh volcanic rock could also soak up carbon dioxide as it weathered, taking a heat-trapping gas from the air.
Scientists still argue about how complete the ice cover was. Seaweed fossils in China hint at livable coastal water during the later freeze. Open water or thin ice could have let sunlight through in places.
Volcanoes kept adding carbon dioxide while ice covered much of the rock that removed it. Heat built up and a thaw could begin. Pale rock above glacial rubble marks that change. Earth went through major freezes twice; why their lengths differed so much is still open.
