“Largest” isn’t a meaningful measure for caves, so these are ranked by surveyed 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:

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