Mammoth Cave, about 90 miles south of Louisville, Kentucky, is a gigantic system of underground tunnels. With names like River Styx, Fat Man’s Misery, and Great Relief Hall, there’s plenty to see in its 425 miles. No other cave on earth comes close to its size… and more than 130 different kinds of animals live in the cave, including the protected Kentucky Cave Shrimp.

“Largest” isn’t a meaningful measure for caves, so these are ranked by passage length. Passageways are still being discovered in some of these caves, so the figures change as exploration continues. Here are the ten longest known caves, measured in miles and kilometers:

1. Mammoth Cave, Kentucky (US): 426 miles (685 km)
2. Ox Bel Ha (Mexico): 336 miles (541 km)
3. Shuanghedong (China): 274 miles (440 km)
4. Sac Actun Dos Ojos (Mexico): 240 miles (387 km)
5. Jewel Cave, South Dakota (US): 220 miles (354 km)
6. Wind Cave, South Dakota (US): 168 miles (270 km)
7. Clearwater Cave (Malaysia): 166 miles (268 km)
8. Optymistychna Cave (Ukraine): 164 miles (264 km)
9. Lechuguilla Cave, New Mexico (US): 152 miles (244 km)
10. Sistema del Alto Tejuelo (Spain): 135 miles (218 km)