About 650 million years ago, something froze the entire planet, coastlines to equator, into a single sheet of ice and snow. This episode of the Catastrophe series follows the scientists testing that idea, starting in the Australian Outback, where geologists find dropstones, fragments carried and dropped by melting ice, sitting in what is now desert. Geobiologist Joe Kirschvink, who coined the term Snowball Earth, tracks the same signature in California's Death Valley, where magnetic readings in the rock date the freeze and place the site near the ancient equator at the time. "The entire Earth would have looked like Antarctica looks today," he says. The film also takes cameras to Alaskan glaciers for present-day comparison, and lays out the case against a partial ice age like those we know: this one, the theory holds, was triggered by a collapse in atmospheric carbon dioxide severe enough to cover nearly the whole surface, not the usual third. It closes on the harder question, how anything survived the deep freeze at all, and what a repeat would mean now.