Yellowstone was the first national park in the world, designated by the U.S. government in 1872.

Before then, it was travelled by explorers and fur trappers. And for 11,000 years before that was a hunting and camping ground for indigenous tribes.

But perhaps the most fascinating part of its history occurred when it was formed.

When you visit Yellowstone, you’re standing above a geologic hot spot in the Earth’s mantle.

Two million years ago, this hot spot created supercharged lava, in magma chambers that eventually erupted, launching volcanic material all the way to the Mississippi River.

The empty chambers then collapsed, forming the first Yellowstone Caldera, a 30-mile-wide shallow crater-basin.

Two more volcanic eruptions happened, 1.3 million and 600,000 years ago, such that three overlapping calderas about 45 miles wide form the center of Yellowstone Park.

Though the calderas have gradually filled with sediment, they’re still home to more than half of the world’s geysers and natural hot spring pools.

Except for its remarkable hydrothermal activity, Yellowstone has been volcanically quiet for 70,000 years—and scientists don’t expect it to erupt again in our lifetimes. Some think it may never.

Still, the park remains one of the most heavily instrumented and closely monitored sites in the world, a geologic wonder that amazes scientists—and millions of visitors each year.