Cave of Swallows (Mexico)
Located in the state of San Luis Potosí, Mexico, this pit cave is popular with ornithologists and adrenaline junkies. The former come to marvel at the clouds of birds – swifts to be exact – clinging to the cave walls, which also serve as breeding ground for parrots and parakeets. The latter base jump 330 meters down the perfectly vertical chamber to the guano-caked ground below. The view from the bottom is as daunting as it is feathered.