In this photograph taken on February 18, 2016, Pakistani children ride on swings in the predominantly Pashtun Korangi District of Karachi. In a rundown district of Karachi, Rabia balks at a neighbour's proposal to vaccinate her children, demonstrating one of the biggest hurdles to eradicating polio in Pakistan by the end of the year: confused and frightened parents. (Photo by Asif Hassan/AFP Photo)

People wash in water from a broken water pipeline on the outskirts of Kolkata, India, March 30, 2016. (Photo by Rupak De Chowdhuri/Reuters)

An employee sorts waste at the Wecycler recycling centre in Ebutte Meta district in Lagos, Nigeria July 28, 2016. (Photo by Akintunde Akinleye/Reuters)

Women dressed as nuns pose for pictures during Halloween celebrations in the Shibuya district in Tokyo, Japan October 31, 2015. (Photo by Thomas Peter/Reuters)

The Canary Wharf financial district stands in London, Britain on August 11, 2019. (Photo by Simon Dawson/Reuters)

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.

Snowfall views in Ercis district of Van, Turkiye on January 14, 2022. (Photo by Necmettin Karaca/Anadolu Agency via Getty Images)

American actress and singer Hailee Steinfeld is seen in the Meatpacking District on December 14, 2019 in New York City. (Photo by Gotham/GC Images)