Migrants ride on top of a northern bound train toward the US-Mexico border in Juchitan, southern Mexico, Monday, April 29, 2013. Migrants crossing Mexico to get to the U.S. have increasingly become targets of criminal gangs who kidnap them to obtain ransom money. (Photo by Eduardo Verdugo/AP Photo)

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 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)

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 voodoo priest smokes in the face of a woman as she holds a cup on her head during a voodoo ceremony in honor of Kouzen Zaka, also known as St. Isidro, in Mexico City, May 2, 2015. According to Haitain voodoo, Kouzen Zaka is the patron of work, whose patronage will help with employment and safeguard crops from robbers. (Photo by Edgard Garrido/Reuters)

Clowns take part in the annual pilgrimage to the Basilica of Our Lady Guadalupe to pay homage to the Virgin of Guadalupe in Mexico City December 16, 2014. Dressed in colourful costumes, hundreds of clowns from all over Mexico walked towards the Basilica to give thanks to the virgin for the work they have received. (Photo by Carlos Jasso/Reuters)

A cat sleeps between customers inside “Meow” cafe, where diners can play, interact or adopt cats given away by their former owners or rescued from the streets, in Monterrey, Mexico, May 14, 2016. (Photo by Daniel Becerril/Reuters)