* Elliptical cave-mouth: 160 by 205 feet (49 by 62 m) wide
* Cave shaft main body: 995 feet (303m) by 440 feet (135m) wide
* Cave's depth (shortest side): 1,094-foot (333 m)
* Cave's depth (highest side): 1,220-foot (370 m)
In Mexico, it is called Sótano de las Golondrinas (Basement of Swallows), because of the birds which make their home there. Unless there are extreme sports people demonstrating their skills, then the birds are the most popular creatures to watch. The pit is so large that it is difficult to gain a sense of its proportions, without seeing the birds fly into it. They freefall, tucking back their wings, so they can plummet into the depths. Then, as they draw near to their nests, their wings are extended. They glide into their perches and rest.
Literally thousands of birds live in this shaft. The vast majority are members of the swift family, hence the name 'Cave of Swallows', though there are also other species. These include the rare cave parrot (cotorras de la cueva) and green parakeets.
Each morning, there is a wondrous spectacle, as the cave's feathered inhabitants flock en masse out of the entrance. The cave's mouth darkens and the sound of their wings is thundrous. It can take up to half an hour to get them all clear; then the sight is repeated, in the evening, when they all come home. Meanwhile, the parakeets spiral out. They emerge as lonely green dots, drifting around the interior far below, coming closer, until they finally reach the top and fly to the skies.
So many birds have lived in this cave, for so long, that the bottom of the pit is spongy to the touch. It is their waste that has built up to carpet the ground. They make their nests amidst the flora that eeks out an existance in the darkened crags. Also at the bottom of the pit are other creatures. Millipedes, scorpions, snakes and all manner of insects. The abyss is a vibrant hub of life, if viewed with the right kind of eyes.
The Cave of Swallows is near to the town of Aquismón, in the state of San Luis Potosi. The local Huastec people viewed it as an entrance to the Underworld. The first foreign explorers were Americans. Three cavers, from Texas, named T.R.Evans, Charles Borland and Randy Sterns, entered the pit on 27th December 1966. They didn't make it all of the way to the floor; returning in 1969 to do that.
Their reports saw interest growing, in the giant hole, throughout the rest of the world. The resulting tourism has changed Aquismón from a small, jungle village, into a thriving hub of amenities. It takes just 20 minutes to walk from the town to the cave. For many, the great attraction are the extreme sports to be had there.
Rock-climbers, cavers, rappellers and BASE Jumpers have all flocked to this site, including one balloonist, who managed to successfully fly a normal sized hot air balloon to the floor of the pit. It can take 10 seconds to BASE Jump down; twenty minutes to abseil; or 1 to 2 hours to climb. Temperatures in the pit can also make life difficult, damaging equipment with its extremes or exhausting the wary. Even highly experienced thrill-seekers and explorers have warned about the Cave of Swallows. (For an example, see this thread, on Cavechat.org.)
However, the sheer number of people visiting, to dive into the cave, has resulted in some restrictions being placed. All sports are banned during the time when the birds are flocking in or out. Also no damage may be done to their nests. The descent into the cave may be made at only one designated spot. Nothing may be thrown into the cave nor left in there afterwards. Helicopters may not be flown near to it. People may not shout into the pit (the echo was great!), as this disturbs the creatures living in there. All of these restrictions came after ornithologists realised how much the bird population was starting to decrease, due to the effect of adventure tourism.