When you take a look at the world on a map, it seems there isn’t much left to discover. There are, of course, the depths of the ocean that are still largely unexplored and perhaps some parts of the poles haven’t yet been seen by human eyes, but other than that it seems that we’ve found almost everything else—until you consider the caves.

These structures are incredible feats of nature and man and many of them are still being discovered today. No one found the Cave of Crystals until 2000 and no one has been able to explore the whole interior due to its extreme heat and humidity. Better yet, the largest cave system in the entire world was just discovered in 2009—and there could be an even larger cave out there, no one knows for sure. These incredible earthly features are being unveiled and explored all the time.

Though you may not be able to visit and get inside some of these caves, they are definitely worth a look. From Patagonia to China, these natural and man-made caves are unbelievable. Check out some of the coolest caves in the world.