700 million years ago Earth froze over completely and stayed that way for millions of years.

How did this deep freeze start, and how did it end? We think it went like this:

At the time, all of Earth’s land was held in one large continent called Rodinia. As it began to break up, huge areas eroded, pulling carbon dioxide from the air and sequestering it in minerals.

With CO2 dropping, the atmosphere cooled, and ice began forming in the polar oceans. As the ice sheets extended, their white surfaces reflected sunlight, which amplified the cooling.

Soon the oceans were covered in ice, and Earth was frozen.

The average surface temperature dropped to negative 60 degrees Fahrenheit.

But beneath the ice, in what some believe to be the birth of plate tectonics, Earth was stirring.

Rodinia kept pulling apart. Volcanos formed along its rift zones and began pumping out carbon dioxide and water vapor.

Buildup of these greenhouse gases eventually drove runaway heating, which melted the glaciers.

In just a few hundred years, global average temperatures rocketed to more than 120 degrees.

The heat restarted the water cycle and drove weather systems that caused erosion, which consumed carbon dioxide. And Earth cooled again. 

Thus began the constant cycling of Earth’s climate—and shortly thereafter, the explosion of life.