It’s a world hard to imagine. Some 650 million years ago, Earth’s land and oceans were almost completely covered by ice and snow. The planet’s population – primitive plants and animals like algae and bacteria – sheltered themselves around hot springs on the ocean floor, in surface ponds melted by volcanic heat, or in nooks where the ice was thin enough for the sun to seep through.

A theory that has been around since 1992 proposes that the Earth was like this at least twice between about 700 million and 600 million years ago. At first geologists dismissed it as a crazy idea, but for the past 10 years the idea of a snowball Earth has gained a lot of respect, if not snowballed.

Paul Hoffman, one of the pioneers of the theory and Sturgis Hooper Professor of Geology at Harvard University, recently uncovered some fascinating details of what happened to Earth’s climate as its global ice covering gave way to rising temperatures. He and Phillip Allen, a geologist based in Switzerland, found evidence of huge rises in sea level coupled with extreme winds and waves. Such storms left giant ripples still visible in Australia, Brazil, Africa, Canada, and islands in the North Atlantic.

Tsunami, hurricanes, and ripples

Continents were not as spread out then as they are now. There may have been only one gigantic land mass, surrounded by oceans and located mainly in the Southern Hemisphere. Sustained winds of 40 mph and higher drove ocean water ahead of it and sent waves as high as 60 feet crashing on the shores. That piled loose sediments into giant ripples in the surrounding seabeds.

Those ripples are still preserved today in rocks that date back some 635 million years. In the eons since, their sharp crests and round troughs have been hardened into rocks and lifted above sea level by the movement of continents and ocean floors.