Artists perform during the opening show directed by German director Volker Hesse, on the opening day of the Gotthard rail tunnel, at the fairground Rynaecht at the northern portal in Erstfeld, Switzerland, Wednesday, June 1, 2016. The construction of the 57 kilometer long tunnel began in 1999, the breakthrough was in 2010. After the official opening on June 1, the commercial operation will start in December 2016. (Photo by Ruben Sprich\Pool Photo via AP Photo)

Dancers from Xinjiang delegation perform during the Opening Ceremony for the 9th National Traditional Games of Ethnic Minorities of the People's Republic of China on September 10, 2011 in Guiyang, China. (Photo by Feng Li/Getty Images)

In a photo taken on April 2, 2020 a staff member feeds a raccoon at the Table A Raccoon Cafe in Seoul. Business has been devastated by the coronavirus outbreak, with South Koreans staying at home under social distancing guidelines, and tourism disappearing. But unlike other firms, animal cafes have to stay open so that staff can look after their stock. (Photo by Ed Jones/AFP Photo)

Customers take photos of a snake at the Tokyo Snake Center, a snake cafe, in Tokyo's Harajuku shopping district August 14, 2015. The cafe has on display 35 non-venomous snakes of 20 different breeds. Open six days a week, the cafe allows customers to enjoy food and beverages while watching a small snake slither around in a cage on their table for a 1,000 yen (8.04 dollars). By paying an additional fee, customers get to hold and pet the snake. (Photo by Toru Hanai/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.

People sit at an oversize picnic table in Soho Square in London May 19, 2014. (Photo by Luke MacGregor/Reuters)

Close up of a couple, with the woman holding the man's knee, underneath a table. (Photo by Peter Dazeley/Getty Images)

A man dressed in traditional clothes tries to pull his opponent over the table at the 40th Alpine Country Championships in Fingerhakeln_finger wrestling_ in Woernsmuehl, Germany, Thursday, May 25, 2017. Competitors battled for the title in this traditional rural sport where the winner has to pull his opponent over a marked line on the table. (Photo by Matthias Schrader/AP Photo)