Sarah Kaplan, The Washington Post: About 700 million years ago, Earth turned into a snowball. The polar ice sheets expanded until they engulfed the globe. The oceans turned to slush. The vast expanses of ice and snow reflected the sun's light back into space, exacerbating the endless winter. Temporary relief came in the form of massive volcanic eruptions, which spewed carbon dioxide into the atmosphere and triggered a period of global warming. But that, too, spiraled out of control. Earth became a greenhouse — its oceans hot enough to cook their inhabitants, its blighted landscape further ravaged by floods. Then, suddenly, something about the shifting continents or ash-darkened skies prompted the planet to cool again. The snowball returned.