Revelers march during the Greenwich Village Halloween Parade, Tuesday, October 31, 2017, in New York. (Photo by Andres Kudacki/AP Photo)

A Mexican wearing a period costume re-enacts the battle of Puebla, in the Penon de los Banos neighbourhood of Mexico City, Mexico May 5, 2016. The battle marked the defeat of French forces by Mexican troops and local Indians in the central state of Puebla in 1862. During the re-enactment, participants fired homemade shotguns loaded with gunpowder and hundreds of men dressed as Indian peasants fought mock battles against others dressed as French invaders. (Photo by Henry Romero/Reuters)

A dog dressed as UPS worker is seen during the 28th Annual Tompkins Square Halloween Dog Parade at East River Park Amphitheater in New York on October 28, 2018. (Photo by Gordon Donovan/Yahoo News)

Donny Golden shares a laugh with dancers from the Irish American Society during the 250th annual St. Patrick's Day parade March 17, 2011 in New York City. The parade honors the patron saint of Ireland and marched for the first time in the U.S. on March 17, 1762, fourteen years before the Declaration of Independence.

Alexa Cinton marches up Fifth Avenue in the Puerto Rican Day Parade on June 12, 2011 in New York City. The Puerto Rican Day Parade draws hundreds of thousands and was first celebrated in New York City in 1958. (Photo by Mario Tama/Getty Images)

Kat Von D attends her first solo art show “New American Beauty” at Sephora Antara store on March 1, 2012 in Mexico City, Mexico. (Photo by Victor Chavez/Getty Images For Sephora)

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.