A hat and boots belonging to a volunteer with the Israeli Antique Authority are seen inside the Cave of the Skulls, an excavation site in the Judean Desert near the Dead Sea, Israel June 1, 2016. A team of volunteers and archaeologists are searching for remains of The Dead Sea Scrolls, one of the oldest known Hebrew texts, in a national bid, initiated by the Israel Antiquities Authority (IAA), to prevent the robbery of such important artefacts and once found to hand them over to the state for preservation. (Photo by Ronen Zvulun/Reuters)

A Malaysian Hindu devotee (C) reacts in a state of trance as she walks towards the Batu caves temple during the Thaipusam festival celebrations in Kuala Lumpur on January 24, 2016. More than a million Hindus celebrate the festival of “Thaipusam” at temples across the country. (Photo by Manan Vatsyayana/AFP Photo)

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)

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.

Vietnam’s Son Doong cave, the largest in the world, could hold a 40-story skyscraper inside. The pristine ecosystem has its own river and jungle. Despite its size, Son Doong wasn’t discovered until 1991. It was lost again for nearly two decades and was fully explored for the first time in 2009. (Photo by Jason Speth/HuffPost)

A pilgrim prays at Mount Al-Noor, where Muslims believe Prophet Mohammad received the first words of the Koran through Gabriel in the Hera cave, ahead of the annual haj pilgrimage in the holy city of Mecca, Saudi Arabia September 7, 2016. (Photo by Ahmed Jadallah/Reuters)

Santiago Eneme (second left) and his Equatorial Guinea teammates can’t watch during their last 16 penalty shoot-out with Mali, although they did go on to win at Limbe Omnisport Stadium in Limbe, Cameroon on January 26, 2022. (Photo by Thaier Al-Sudani/Reuters)