Cave of Swallows (Mexico)
Credit: Onixmtz / Wikipedia

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.