More than 600 million years ago, ice sheets covered our planet on both land and sea, with temperatures dipping to -130 degrees celsius. And there was no escaping the cold at the equator, where temperatures fell below zero!

Amazingly, this happened twice in the Cryogenian Period. So what happened? “The most popular theory is that our planet thermostat just failed,” says PBS Eons narrator Kallie Moore. “That thermostat is the carbon cycle, the flopping back and forth of carbon between the atmosphere and the Earth’s crust, and it starts with volcanoes.” Tune in to learn the details.