The Cave of Swallows, also called Cave of the Swallows (Spanish: Sótano de las Golondrinas), 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 meters wide and is undercut around all its perimeter, further widening to a room approximately 303 by 135 meters wide. The floor of the cave is a 333-meter freefall drop from the lowest side of the opening, with a 370-meter 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.
Opened up by water erosion in a fault on an impermeable limestone plain and with a roughly conical shape, the cave has been known to the local Huastec people since ancient times. The first documented exploration was on 27 December 1966 by T. R. Evans, Charles Borland and Randy Sterns.
Temperatures in the cave are low. Vegetation grows thickly at the mouth, where rains can cause waterfalls cascading into the cave. The cave floor is covered with a thick layer of debris and guano on which “millipedes, insects, snakes, and scorpions” live. There is also a narrow sinkhole in a fault of lower Cretaceous limestone which goes down at least a further 512 m.