Earth has experienced dozens of ice ages in the past few million years. At the coldest point of the most recent one, ice sheets over a thousand feet thick covered much of North America.

Then, 15,000 years ago, the world began to warm, and ice started melting—until 13,000 years ago, when Earth plunged back into the Ice Age for a thousand more years.

Scientists thought this may have been caused by trillions of gallons of cold glacial meltwater flooding out into oceans, interrupting the warmer surface currents that heat the continents.

But recently, they found platinum in sediments in several places around the world, all correlating to that time when Earth began cooling again. Some thought this may have been caused by a meteorite impact dispersing platinum into the atmosphere.

While some meteorites contain platinum, most are also high in iridium. But they found no iridium in these deposits.

A debate began, sending teams to analyze sediments around the world. Many of them showed a platinum spike from that same period.

The search is now on for the impact zone of a huge, rare magmatic iron meteorite, which contains little iridium. If and when they find it, you’ll hear about it on a future EarthDate.

After that 1200-year cold snap, Earth warmed dramatically—in as few as 40 years. And stable mild temperatures for the last 12,000 years have allowed the flourishing of all civilization on Earth.