Human-made monuments may be impressive, but it’s hard to beat nature’s creations, especially caves. Between 10 and 15 million years ago, rainwater began percolating through the soil of what’s now Kentucky. Along the way, the water picked up carbon dioxide and became acidic. The acidic water squeezed through cracks in the area’s bedrock, causing the rock to dissolve. Openings in the rock formed and then grew very slowly over time until they became what’s now known as the Mammoth Cave system. 

There are caves all over the world. Some of them form in the way we just described, while others result from earthquakes or the flow of molten lava. But what makes Mammoth Cave unique is that it’s the world’s longest known cave. So far, explorers have mapped 426 miles of passages—nearly as long as the distance from Philadelphia, Pennsylvania, to Cleveland, Ohio—and there’s a lot more left to explore.

Inside the cave, explorers have found clues to North America’s past. They’ve collected artifacts left behind by prehistoric Indigenous people. The cave is also full of fossils—even shark fossils dating from more than 300 million years ago, when there was a sea where Kentucky is today.

Mammoth Cave passages occur in a variety of shapes and sizes and are rife with geologic formations—signs that the cave system is still forming and growing. Check out the slideshow for some images of Mammoth Cave.