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Photo: Onixmtz, CC BY-SA 3.0 via Wikimedia Commons
This is not a pre-packaged cavern tour, kids. This is an adrenaline blast for caving and climbing junkies. It is a Karst cave entered through a ridiculously long rappel down a straight drop of 333 meters (1,092 feet). You'll be rapelling down for at least an hour, in the dark (to repeat, this is no packaged tour.) The cave gets its name from all these flocking birds that nest in the cave walls. None of the birds are swallows, but there are lots of swifts and green parakeets.
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.
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Cascada de Tamul
Spectacular large waterfall on the Rio Santa Maria comprising a series of adjacent drops of 105 meters to the river below.