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 pilot inflates a hot air balloon while taking part in the 38th International Balloon Festival of Chateau-d'oex, Switzerland, 23 January 2016. The festival takes place from 23 to 31 January. (Photo by Vincent Vannay/EPA)

A volunteer of District Magistrate (DM) office dressed as Yamraj, or Hindu God of death, stops people for not wearing masks, amidst the spread of the coronavirus(COVID-19) disease, in New Delhi, India, September 28, 2020. (Photo by Anushree Fadnavis/Reuters)

A demonstrator smashes a shop window during a protest condemning the arrest of rap singer Pablo Hasél in Barcelona, Spain, Saturday, February 20, 2021. A fifth night of peaceful protests to denounce the imprisonment of a Spanish rap artist once more devolved into clashes between police and fringe group members who set up street barricades and smashed storefront windows in Barcelona. (Photo by Joan Mateu/AP Photo)

Burka-wearing Afghan women walk past a clothing shop, in Herat, Afghanistan, Tuesday, November 23, 2021. (Photo by Petros Giannakouris/AP Photo)

A runner participates in the Santa Run 10K race in Caracas, Venezuela, Sunday, December 19, 2021. This is the first Santa Run since the start of the pandemic. (Photo by Ariana Cubillos/AP Photo)

A woman poses for a picture near an ice hole as a man takes a dip during celebrations of the Orthodox Christian feast of Epiphany in the settlement of Ivanovskoye in the Moscow region, Russia on January 19, 2022. (Photo by Maxim Shemetov/Reuters)