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 voodoo priest smokes in the face of a woman as she holds a cup on her head during a voodoo ceremony in honor of Kouzen Zaka, also known as St. Isidro, in Mexico City, May 2, 2015. According to Haitain voodoo, Kouzen Zaka is the patron of work, whose patronage will help with employment and safeguard crops from robbers. (Photo by Edgard Garrido/Reuters)

Vehicles and weapons of the Justice and Equality Movement (JEM) rebels are seen on display, after victory celebrations by the Sudanese Armed Forces (SAF) and the Rapid Support Forces (RSF), in Niyala Capital of South Darfur, May 4, 2015. (Photo by Reuters/Stringer)

A member of the “Los Danzantes y Los Boteiros” folk group performs during the parade of the 10th International Festival of the Iberian Mask in Lisbon, Portugal May 9, 2015. The festival runs until next Sunday and about 690 masked people from Spain and Portugal are participating, according to organizers. (Photo by Rafael Marchante/Reuters)

Masked penitents holds their crosses during spring “Romeria Cruceros de Arce”, in Roncesvalles, northern Spain, Sunday, May 10, 2015. Every year on the second Sunday in spring, people with crosses march from their small Pyrenees towns to Roncesvalles Church in tribute of the Virgin. (Photo by Alvaro Barrientos/AP Photo)

Model Karlie Kloss walks on the red carpet as she arrives for the opening ceremony and the screening of the film “La tete haute” out of competition during the 68th Cannes Film Festival in Cannes, southern France, May 13, 2015. (Photo by Benoit Tessier/Reuters)

Children react after what activists said was shelling by forces loyal to Syria's President Bashar al-Assad near the Syrian Arab Red Crescent center in the Douma neighborhood of Damascus May 6, 2015. (Photo by Bassam Khabieh/Reuters)

Women spray their designs on a wall during a graffiti class offered by the LATA 65 organization in Lisbon, Portugal May 14, 2015. The LATA 65 organization is an initiative for the elderly in the area of urban art. Since it began in 2012, they have introduced the world of graffiti to over 100 senior citizens, giving workshops in different neighborhoods of Lisbon. (Photo by Rafael Marchante/Reuters)