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)

Cannabis cigarettes at the growing facility of the Tikun Olam company on March 7, 2011 near the northern city of Safed, Israel. In conjunction with Israel's Health Ministry, Tikon Olam are currently distributing cannabis for medicinal purposes to over 1800 people in Israel.

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.

A group of scantily-clad women pose for photographs at the annual Venus erotic trade fair in Berlin, Germany on 12 October 2017. Venus Berlin is among the largest international erotic trade fairs, with more then 250 exhibitions from 40 countries and 30,000 visitors. (Photo by Alamy Live News)

Edibles are displayed at Shango Cannabis shop on first day of legal recreational marijuana sales beginning at midnight in Portland, Oregon October 1, 2015. The sale of marijuana for recreational use began in Oregon on October 1, 2015 as it joined Washington state and Colorado in allowing the sale of a drug that remains illegal under U.S. federal law. (Photo by Steve Dipaola/Reuters)

In this September 19, 2014 photo, explosives are detonate by Peruvian counternarcotics forces on a part of a clandestine grassy airstrip in the Apurimac, Ene and Mantaro River Valleys, or VRAEM, the world's No. 1 coca-growing region, in Ayacucho, Peru. The dynamiting of craters by Peruvian security forces into clandestine airstrips cuts into profits but hardly discourages cocaine traffickers who net tens of thousands of dollars with each flight flown from these airstrips. (Photo by Rodrigo Abd/AP Photo)

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)

The photographer holds up a Samsung Galaxy Tab 7.7 tablet at the Samsung hall at the IFA 2011 consumer electonics and appliances fair the day before the fair's official opening on September 1, 2011 in Berlin, Germany. The IFA 2011 will be open to the public from September 2-7. (Photo by Sean Gallup/Getty Images)