Known as Basement of the Swallows in Spanish and the Cave of Swallows in English, this cave shaft is an elliptical mouth to a drop below that measures 49 by 62 meters, although the shaft itself widens to approximately 303 by 135 meters. The floor of the cave is a 333-meter freefall drop from the lowest side of the opening. It is the largest known cave shaft in the world and the 11th deepest.

The cave has seen an increase in tourism and popularity as it appeals to adrenaline junkies and extreme sports enthusiasts. It has also become a mecca for base jumpers and vertical cavers. Extreme sports tourism has dramatically increased visitation to the cave, who are working to become the most well-known aerial inhabitants of the cave. Another activity popular here is rappelling, which can take an hour or more and is fairly stressful on the rope and equipment and must be undertaken with a spray bottle to combat heat buildup. The ascent out of the cave is, of course, more arduous than the journey down and can take two hours. A more unique type of adventure can be had with an average-sized hot-air balloon that navigates the cave opening, successfully landing on the cave floor.

The birds themselves remain a dramatic and beautiful sight and are the source of the cave’s name. Although actual swallows are rarely found there, the other birds behave in a quite spectacular fashion. At sunrise and sunset, when the birds exit and enter the cave, two beautiful behaviors can be observed. Early morning, they leave the cave mouth, flying in concentric circles and gaining height until they burst free and fly off into the sky. When they return, however, they perform an even more daring feat and predate the base jumpers in doing so. After crossing the edge, they fold in their wings and essentially free-fall until they reach the height of their nets. Watching this is always popular with tourists.

The cave temperatures remain quite low and the vegetation is thick and lush at the cave mouth. The bottom is less pretty, as it is smeared thick with the debris and guano from its many inhabitants .

The cave takes its name from the Huastec people, who have known of it since ancient times. The first documented descent, however, didn’t take place until December 1966, when T.R. Evans, Charles Borland, and Randy Sterns entered the cave.