The Cave of Swallows, also called 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 160 by 205 feet (49 by 62 m) wide and is undercut around all its perimeter, further widening to a room approximately 995 feet (303m) by 440 feet (135m) wide.
The floor of the cave is a 1,094-foot (333 m) freefall drop from the lowest side of the opening, with a 1,220-foot (370 m) 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. A skyscraper such as New York City's Chrysler Building could easily fit wholly within it.
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 (vencejos in Spanish) and green parakeets (periquillo quila).Each morning, flocks of birds exit the cave by flying in circles, gaining height until they reach the opening and the sky.
In the evenings a large flock of swifts circle the mouth of the cave and about once each minute, a group of perhaps fifty breaks off and heads straight down towards 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.