El Sótano de las Golondrinas, (the Cave of Swallows), in Mexico, is the largest known cave shaft in the world. A 200 foot wide hole in the forest floor drops straight down 1,220 feet to the cave floor below.
The natural wonder attracts thousands of cavers from around the planet every year. The shaft is so deep that clouds form inside it.
These photographs were taken by Dave Bunnell, 61, who has been caving for 40 years. Having travelled to 18 different countries to test out their finest rocky offerings, his favourite is El Sotano de las Golondrinas.
The cave used to be popular with BASE jumpers, but ornithological studies have found that the bird population of El Sótano de las Golondrinas is decreasing. To limit disturbance, access and activities are more tightly controlled, and BASE jumping is no longer permitted.
The pit was first descended in December of 1967 by a trio of Americans and since then cavers from all over the world have visited it.The cave's total depth is 1,638ft with the final few hundred feet accessible via a pit series called The Crevice.
It is believed that the sinkhole formed from the collapse of the roof of an underground cave, which in turn was formed by water erosion along a fault line in the limestone.
Flocks of green parakeets and white-collared swifts leave the cave each day, circling up the shaft before pouring out into the surrounding jungle. This is the origin of the cave's name: El Sótano de las Golondrinas, (Cave of Swallows).
A hot air balloon was once navigated through the 160-foot wide opening, landing on the floor below.
The floor of the cave is rich with plant life. The thick layer of debris and guano on the floor is also home to millipedes, insects, snakes, and scorpions.
A climber waits at the mouth of the cave whilst someone is lowered. The higher side of the shaft mouth is thick with foliage, so most cavers most fix their ropes to the lower side, where bolts have been fixed into the rock and the area is clear of obstructions.
Waiting at the cave entrance. The journey back up takes between 40 minutes and 2 hours.
A view from the entrance of the Golondrinas
