A visitor walks underneath a replica of a Titanosaur at the Cretaceous park in Cal Orcko, on the outskirts of Sucre, Bolivia, July 22, 2016. A footprint measuring over a meter wide that was made by a meat-eating predator some 80 million years ago has been discovered in Bolivia, one of the largest of its kind ever found. (Photo by David Mercado/Reuters)

Ethnic Uyghur tourists guides wait to perform in a show for Chinese tourists on June 29, 2017 in the old town of Kashgar, in the far western Xinjiang province, China. Kashgar has long been considered the cultural heart of Xinjiang for the province's nearly 10 million Muslim Uyghurs. (Photo by Kevin Frayer/Getty Images)

Eritrean refugee children play at Mai-Aini refugee camp near the Eritrean boarder in the Tigrai region in Ethiopia February 10, 2016. Each month as many as 5,000 people flee Eritrea according to U.N. figures, estimates the Eritrean government disputes. The government puts the population at about 3.6 million, while other estimates suggest it could be almost double that. (Photo by Tiksa Negeri/Reuters)

Labourers use a crane to lift the hand of a giant bronze statue of former King Rama I at Ratchapakdi Park in Hua Hin, Prachuap Khiri Khan province, Thailand, August 4, 2015. The park is being constructed by the Thai army to honor past Thai monarchs and is situated on an army compound near the Klai Kangwon Palace. The project is estimated to cost about 700 million baht ($20,000,000), according to local media. (Photo by Athit Perawongmetha/Reuters)

School children celebrate after being rewarded for their dance performance during India's Independence Day celebrations in Chandigarh, India, August 15, 2015. Indian Prime Minister Narendra Modi's independence day speech focused on measures his “Team India” had rolled out to include millions of poor Indians in the banking and insurance systems, policies for workers and farmers and successes in the fights against inflation and corruption. (Photo by Ajay Verma/Reuters)

British visitors wearing Bavarian costumes walk ahead the opening of the traditional Bavarian Oktoberfest festival at the Theresienwiese in Munich, southern Germany, on September 20, 2014. Germany's world-famous Oktoberfest kicks off with millions of revellers set to soak up the frothy atmosphere in a 16-day extravaganza of lederhosen, oompah music and, of course, beer. (Photo by Karl-Josef Hildenbrand/AFP Photo/DPA)

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.

Women perform square dancing at a park square in Beijing, China, April 9, 2015. Chinese-style square dancing is running circles around other hobbies for the elderly, with millions taking to parks and plazas across China, swivelling their hips and rolling their arms – but it's not to everyone's liking. (Photo by Kim Kyung-Hoon/Reuters)