Ice swimmer Klaudia prepares to swim in a filled up water kettle in an ice cave inside the Nature Ice Palace, with a hight of 3,250 meters (10,663 feet) above sea level, at Hintertux Glacier near Hintertux, some 480 kilometers (298 miles) western of Vienna, Austria, 28 July 2018. (Photo by Christian Bruna/EPA/EFE)

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)

Storm erupts over Pacific Ocean as plane zooms past at 37,000 ft. The amazing picture shows a massive cloud erupting against a starry night sky as the off-duty airman Santiago Borja flew over the Pacific near Panama City. He had just moments to take the breathtaking snap as the Boeing 767 zoomed by at 37,000 ft on its way from North America to Ecuador. He also had take the picture perfectly in time with the flash of lightning to capture the storm in all its glory. (Photo by Santiago Borja/Solent News)

Mateo Santiago, Field Manager at Tropical Bamboo Nursery and Gardens, photographs rain water collecting in the corpse flower as Garden Manager Melanie Benson steadies a ladder. Santiago crinkled his nose up distastefully when he described its odor, which was at its worse Sunday night. “It smelled like a dead rat”. (Photo by Melanie Bell/Palm Beach Daily News)

A student protester throws a stone against a riot police vehicle during a demonstration to demand changes in the education system in Santiago, Chile, April 21, 2016. (Photo by Ivan Alvarado/Reuters)

A demonstrator kicks a riot police vehicle during an unauthorized march called by the Chilean student federations to protest against government's education reform, in Santiago, Chile July 5, 2016. (Photo by Ivan Alvarado/Reuters)