A woman in a USA sash, who would help show U.S. President Barack Obama to his seat, waits with other women in traditional dress for the start of the ASEAN Summit gala dinner in Vientiane, Laos September 7, 2016. (Photo by Jonathan Ernst/Reuters)

Russia's President Vladimir Putin (L) reacts as he talks to Belarus' President Alexander Lukashenko before a meeting on resolving the Ukrainian crisis in Minsk, February 11, 2015. A document seen by Reuters at talks on the Ukraine crisis suggested the sides may agree to end fighting in eastern Ukraine with a ceasefire starting on February 14, the withdrawal of heavy weapons and the creation of a security zone. (Photo by Mykola Lazarenko/Reuters)

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)

In this photo illustration a sat nav is seen attached to a windscreen of a car in front of a sign telling some motorists to ignore their satellite navigation devices on January 6, 2012 in Symonds Yat, England. (Photo illustration by Matt Cardy/Getty Images)

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.

Two attendees compete in the biggest vape cloud competition at the Vape Summit 3 in Las Vegas, Nevada May 2, 2015. According to new research provided to Reuters, youngsters say that performing tricks is one of the top two reasons they began using e-cigs. (Photo by David Becker/Reuters)

A farmer and his cow “Cilly” wait for results of the “Milchgipfel” (lit. Milk summit) which takes place at the German Ministry of Food and Agriculture in Berlin, Germany on May 30, 2016. Minister of Food and Agriculture Christian Schmidt met with representatives of agriculture and dairy farming to evaluate measures to help dairy farmers in light of plummeting milk prices. (Photo by Maurizio Gambarni/EPA)

Security personnel patrol around West Lake in Hangzhou, Zhejiang Province, China, August 31, 2016. (Photo by Reuters/Stringer)