* Astronomy

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 m 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. 
The first documented exploration was on 27 December 1966 by T. R. Evans, Charles Borland and Randy Sterns.
Read more

Spoiler

Spoiler