Cave of Swallows, Show cave in San Luis Potosí, Mexico.

The Cave of Swallows is an open-air pit cave in Aquismón, San Luis Potosí, with an elliptical mouth 49 by 62 meters wide that is undercut around its perimeter. The shaft widens into a room about 303 by 135 meters, and the floor lies 333 meters below the lowest rim. Its walls are banded grey, brown and tan limestone, with green plant tufts in the cracks.

The Huastec people knew the shaft long before outsiders; T. R. Evans, Charles Borland, Randy Sterns and Sid West were first shown it on 27 December 1966. The first documented descent followed on 4 April 1967. A Mexican expedition reached the floor in 1974, when Lorenzo García Gallardo descended with the Grupo México; in 1978 Isabel Vivían became the first Mexican woman to do so.

The Spanish name Sótano de las Golondrinas means "Basement of the Swallows", though the birds nesting in wall holes are mostly white-collared swifts and green parakeets, not true swallows. The Huastec people knew the shaft long before 1966. Each morning the birds spiral upward out of the mouth; at dusk groups of about fifty peel off and free-fall into the opening, a sight tourists come to watch.

Visitors need experienced local guides and specialized equipment for safe exploration of the cave system. The steep descent and ascent require physical preparation and a comfort with heights.

The birds here are white-collared swifts and green parakeets, not swallows, despite the name. Each evening a flock circles the mouth and about once a minute a group of roughly 50 folds its wings and free-falls toward the nests, pulling out of the dive at nest height.