Sixty-six million years ago, three-fourths of all species on Earth died out. Scientists knew it. But they didn’t know why.

Then, in the 1970’s, geologist Walter Alvarez and his physicist father Luis found a layer of unusual clay corresponding to the time of the extinction.

They discovered it contained a high concentration of iridium, a metal that’s very rare on Earth: it comes only from meteorites.

This intrigued other scientists, who looked at rocks from the same geologic horizon in different locations—and found more iridium. They began to suspect that a massive impact may have caused the mass extinction.

In the Caribbean, the clay layer was far thicker. If an event had happened, it must have been nearby.

Decades earlier, the Mexican state oil company had drilled exploratory wells on the Yucatan peninsula, west of Cancun. Rather than oil, they believed they’d hit a buried volcano.

So the scientists reexamined those samples and found the rocks weren’t volcanic after all, but showed signs of a profound impact.

They turned to modern aerial surveys and satellite images, which revealed an enormous circular feature with sinkholes lining its rim.

And they knew they had found it! Directly across the Gulf of Mexico from New Orleans was an ancient impact crater 125 miles in diameter.

This was where an asteroid struck Earth, ending the rule of the dinosaurs.