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.

An activist kicks the shields of the military police officers during a demonstration in the military zone of the 27th infantry battalion in Iguala, Guerrero, January 12, 2015. Activists and relatives of the 43 missing trainee teachers from Ayotzinapa's teacher training college broke into the military zone, located less than a mile from where the students went missing, in an attempt to look for the missing students.The remains of only one of the 43 students has been identified so far. (Photo by Jorge Dan Lopez/Reuters)

A migrant boy sits on railway tracks at a train station in Tovarnik, Croatia, September 20, 2015. (Photo by Antonio Bronic/Reuters)

Buildings in Nogales, Mexico (R) are separated by a border fence from Nogales, Arizona, United Sates, October 9, 2016. Building a wall along the U.S.- Mexico border has been a contentious subject in this year's U.S. presidential election. In parts of California and Arizona, a wall already exists. It runs across rocky deserts, flowing sand dunes and miles of agricultural land. (Photo by Mike Blake/Reuters)

A man dressed up as a "Catrin", a Mexican character also known as "The Elegant Death", is pictured during a Catrinas parade in Mexico City October 31, 2015. (Photo by Carlos Jasso/Reuters)

Cuban Daymara A., 20, poses for a photograph after she crossed the border from Colombia through the jungle into La Miel, in the province of Guna Yala, Panama December 2, 2015. According to local authorities in La Miel, some 100 to 150 Cubans have been entering Panama from Colombia every day for the last three months. Scores of Cubans have come to Panama as they seek overland passage towards the United States fearing a recent detente between Washington and Havana could end their preferential treatment. (Photo by Carlos Jasso/Reuters)

Young migrants gesture during a day of solidarity organised by local organizations and resdients for asylum seekers at a makeshift camp outside the foreign office in Brussels, Belgium September 6, 2015. Over 150,000 people seeking to enter Europe have reached Hungary this year, most coming through the southern border with Serbia, and many apply for asylum but quickly try to leave for wealthier EU countries. (Photo by Yves Herman/Reuters)

A clown plays the violin during the XXI Convention of Clowns, at the Jimenez Rueda Theatre, in Mexico City, Mexico, October 19, 2016. In the wake of the creepy clown sightings in several states of North and South Americas and Europe that caused widespread panic, professional clowns from across Latin America came together at an annual convention in Mexico to give a serious message: “We are clowns, not killers!”. Around 200 clowns took part in the 21st International Clown Convention, dressed in bright coloured clothes and makeup, and chanted the word as they clicked group pictures to counter the “creepy clown” craze. (Photo by Carlos Jasso/Reuters)