The largest cave shaft in the world, home to thousands of birds, and popular with BASE jumpers. Simply staring down into the Cave of Swallows is a test of one’s fortitude. The Cave of Swallows, also called the Cave of the Swallows, is an open air pit cave in the Municipality of Aquismón, San Luis Potosí, Mexico.
The elliptical mouth, on a slope of karst, is 49 by 62 m wide and is undercut around all of its perimeter, widening to a room approximately 303 by 135 meters (994 by 442 ft) wide. The floor of the cave is a 333-meter (1092 ft) freefall drop from the lowest side of the opening, with a 370-meter (1,214 ft) drop from the highest side, making it the largest known cave shaft in the world, the second deepest pit in Mexico and perhaps the 11th deepest in the world.
The cave gets its name from the green parakeets and white-collared swifts that live along its walls. Each day the birds fly in concentric circles up the cave shaft before the flocks come spilling out of the of the hole into the jungle.
The cave has more recently become the home to a number of newer aerial species, namely vertical cavers and base-jumpers. A popular destination with the extreme sports enthusiasts, it is tall enough for the BASE jumpers (BASE stands for Buildings, Antennas, Spans, and Earth, all things they enjoy flinging themselves off of) to leap off the edge and free-fall for a couple of seconds before deploying their parachutes.
While the cave has long been known to the local Huastec people, it was first explored by outsiders in 1966 by T. R. Evans, Charles Borland and Randy Sterns. The cave is so deep that when rappelling, which can easily take an hour, the friction can cause the rope and equipment to heat up to dangerous levels, so spray water bottles are used to cool the equipment as one descends.
According to atlasobscura