Carlsbad Caverns National Park is a very rare place.

Its main room is the largest cave chamber in North America, covering more than eight acres. And its cave formations are unsurpassed anywhere.

It’s also rare because it was created not by carbonic acid percolating down through the soil but by sulfuric acid coming up from below.

Around 20 million years ago, hydrogen sulfide gas began to migrate underground from oil deposits in the Permian Basin of Texas, toward the exposed fossilized reefs near what is now Carlsbad, New Mexico.

There, the hydrogen sulfide gas rose to meet the water table. It combined with oxygen to create sulfuric acid and began to dissolve the limestone that forms the Permian reef.

Over the ages, as the water table rose and fell, the sulfuric acid worked on shallower and deeper layers, creating hundreds of caverns, some thousands of feet below ground.

Around a million years ago, erosion connected the caves to the surface. Mineral-enriched water began to trickle in and, drop by drop, over millennia, formed spectacular crystal formations of calcite, aragonite, and gypsum.

Seven thousand five hundred years ago the area began to dry up and the cave formations stopped growing.

But they’re preserved there today in all their glory—waiting for you to make the trip to Carlsbad Caverns, to see some of the most stunning geology in the world.