Internally displaced Syrian children stand at the entrance of their makeshift shelter that is an underground cave in Om al-Seer, southern Idlib countryside, Syria December 26, 2015. (Photo by Khalil Ashawi/Reuters)

Dancers perform during the Sambodromo do Brasil jungle party in the Cave stage, Elrow Town at Queen Elizabeth Olympic Park on August 18, 2018 in London, England. (Photo by Ollie Millington/Redferns)

Hindu pilgrims walk along a mountain path as they make their pilgrimage to the sacred Amarnath Cave, one of the most revered Hindu shrines, near Baltal, Kashmir, India. (Photo by Daniel Berehulak)

A cenote is a natural phenomenon, a sinkhole in the Earth’s surface. The Yucatan Peninsula in Mexico has an estimated 7,000 cenotes because it is primarily made up of porous limestone. For millions of years, rainfall slowly ate away at the limestone and a huge system of underground caves and caverns was formed. Many filled with water from rain or from the underground water table. When the roof of a water filled cave collapses, a cenote is born. The water found in a cenote may be fresh water, salt water, or both. Structurally it may be completely open, like a lake, almost completely closed with just a small opening at the top, or somewhere in between.

Asia Sawicka as Anastasia IV poses for a photograph prior to a rehearsal of the Circus of Horrors' latest show Voodoo, ahead of Halloween, at the Wookey Hole Caves Theatre near Wells on October 19, 2017 in Somerset, England. (Photo by Matt Cardy/Getty Images)

National Geographic has created “Air, Land & Sea: the 50 greatest wildlife photographs” exhibition. Here: CT4 Crocodile cave on the Salamat river. Set up with Nathan Williamson last chip rain came while we were with the nomads. (Photo by Michael Nichols/National Geographic)

Francisca Gomez stands at an entrance to her home, a cave where she has been living for the past 50 years, in the mountains on the outskirts of Chusmuy, Honduras April 21, 2015. The 75-year old widow receives help from neighbors and friends and she makes a little money by selling wood she collects, according to a local newspaper. (Photo by Jorge Cabrera/Reuters)

Teachers carry children away from Briarwood Elementary school after the tornado destroyed the school in south Oklahoma City. (Photo by Paul Hellstern/The Oklahoman)