Sotano de las Golondrinas (Cave of Swallows) - Huehuetlan, Mexico

The Cave of the Swallows is deep enough to fit the Statue of Liberty twice easily or the Eiffel Tower. It is the deepest cave in the world and is home to hundreds of swallows. It has also become a favourite spot for the bravest BASE jumpers and other extreme sports enthusiasts. They leap off the edge, free-falling for a while before opening their parachutes. It reportedly takes 10 seconds to reach the bottom, but then they have a grueling climb back up to the top which can take up to two hours.