Cave of Swallows

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 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.

Source: Wikipedia