Men dressed as soldiers pose with models before the opening of the Gamescom 2013 fair in Cologne,Germany, on August 23, 2013. The Gamescom convention is Europe's largest video games trade fair. (Photo by Ina Fassbender/Reuters)

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.

A Brazil fan waits for the start of the group A World Cup soccer match between Brazil and Croatia, the opening game of the tournament, in the Itaquerao Stadium in Sao Paulo, Brazil, Thursday, June 12, 2014. (Photo by Felipe Dana/AP Photo)

Guests arrive for the opening ceremony of the 23rd Life Ball in Vienna May 16, 2015. Life Ball is Europe's largest annual AIDS charity event and takes place in Vienna's city hall. The Life Ball is a charity gala to raise money for people living with HIV and AIDS. (Photo by Leonhard Foeger/Reuters)

Russian honour guards wearing protective masks attend the opening ceremony of the 6th International Military Technical Forum “Army 2020” and Army Games in the military Patriot Park outside Moscow on August 23, 2020. (Photo by Kirill Kudryavtsev/AFP Photo)

After a long time renovation project, the old town in Taiyuan city which was founded in 1357, finally lights up and opens to public in Taiyuan City, north China's Shanxi Province, 23 November 2020. (Photo by Rex Features/Shutterstock)

Belgium's Elise Mertens, left, and Aryna Sabalenka of Belarus celebrate with their trophy after defeating Barbora Krejcikova and Katerina Siniakova of the Czech Republic in the women's doubles final at the Australian Open tennis championship in Melbourne, Australia, Friday, February 19, 2021. (Photo by Hamish Blair/AP Photo)

Japan's Naomi Osaka sits with her towel over her head during a break in her third round loss to Coco Gauff of the U.S. at the Australian Open tennis championship in Melbourne, Australia, Friday, January 24, 2020. (Photo by Lee Jin-man/AP Photo)