Cave of Swallows
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 wide and is undercut around all of its perimeter, widening to a room approximately wide. The floor of the cave is a freefall drop from the lowest side of the opening, with a 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 sheer drop in the world.
History
The cave has been known to the local Huastec people since ancient times. T. R. Evans, Charles Borland, Randy Sterns, and Sid West were first shown the cave on 27 December 1966. The first documented descent was on 4 April 1967.Geology
The cave is formed in the El Abra and Tamabra formations, limestones of Middle Cretaceous age. The cave's speleogenesis is still not fully known but is a result of solutional enlargement along a vertical fracture, with subsequent vadose enlargement.Etymology
The cave's Spanish name Sótano de las Golondrinas means Basement of the Swallows, owing to the many birds which live in holes on the cave walls. These are mostly white-collared swifts and green parakeets. Actual swallows are in fact rarely found here.Each morning, flocks of birds exit the cave by flying in concentric circles, gaining height until they reach the entrance. In the evenings a large flock of swifts circles the mouth of the cave and about once each minute, a group of perhaps 50 breaks off and heads straight down toward the opening. When they cross the edge, the birds pull in their wings and free-fall, extending their wings and pulling out of the dive when they reach the heights of their nests. Watching this has become popular with tourists.