It took 10 million years for Kentucky's Green River to create Mammoth Cave, and it shows: Mammoth Cave lives up to its name. It's the longest known cave in the world, by far: Measuring approximately 360 miles (580 kilometers) long, it's four times longer than the second longest cave, Optimisticeskaya in the Ukraine [sources: NPS, GORP]. And that's just the part of Mammoth that's has already been explored. Experts believe the cave might extend 1,000 miles (1,600 kilometers) [source: NPS].
It's not just extreme in length, either. Mammoth Cave boasts some interesting life forms, like eyeless fish, shrimp and beetles as well as spiders with no pigmentation. There are tremendous columns, like the 192-foot (59-meter) Mammoth Dome, and rivers running through the lowest levels of the cave.
Visitors to Mammoth Cave National Park in Kentucky can check out 10 miles (16 kilometers) of the extensive cave system.
Up next: a cave system with more than one world-famous component.