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.

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)

Siberian Huskies pull a contestant during the “Kara-Dag 2016” open amateur dog sled and skijoring race near the village of Yelovoye in Taiga district, outside Krasnoyarsk, Siberia, Russia, February 21, 2016. Dogs and their owners from various regions of Siberia took part in an annual race. (Photo by Ilya Naymushin/Reuters)

A contestant of the World Beard And Mustache Championships poses for a picture during the opening ceremony of the Championships 2015 on October 3, 2015 in Leogang, Austria. Over 300 contestants in teams from across the globe have come to compete in sixteen different categories in three groups: mustache, partial beard and full beard. The event takes place every few years at different locations worldwide. (Photo by Jan Hetfleisch/Getty Images)

Visitors walk past "Sculpture #1" by Darren Bader at the Frieze Art Fair in London, Britain October 14, 2015. The Frieze Art Fair, which is one of the world's leading contemporary art fairs, opens today and runs through October 17. (Photo by Suzanne Plunkett/Reuters)

A woman wearing a traditional "mantilla" dress takes part in an open-air mass to celebrate Madrid's patron saint La Almudena Virgin in Madrid, Spain, November 9, 2015. (Photo by Susana Vera/Reuters)

Visitors walk past a train-shaped ice sculpture ahead of the 31st Harbin International Ice and Snow Festival in the northern city of Harbin, Heilongjiang province, January 4, 2015. The winter festival will be officially opened on January 5, 2015. (Photo by Kim Kyung-Hoon/Reuters)