The Cave of Swallows, the largest known cave shaft in the world, is discovered in Aquismón, San Luis Potosí, Mexico.

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 m wide. The floor of the cave is a 333 m (1,093 ft) freefall drop from the lowest side of the opening, with a 370 m (1,210 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 sheer drop in the world.