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.

The sign above the door of restaurant and hotel “Am Paulusbogen” is partially submerged in the flooded centre of the Bavarian town of Passau, about 200 km (124 miles) north-east of Munich June 3, 2013. (Photo by Michaela Rehle/Reuters)

“NTV (Cyrillic: НТВ) is a Russian television channel. As a subsidiary of Vladimir Gusinsky's company Media-Most, it was a pioneer in the post-Soviet independent television media, but was later taken over by state-owned Gazprom”. – Wikipedia

Photo: A woman with “I love NTV” painted on here face cheers in support of the independent Russian NTV television station April 7, 2001 during a rally in Moscow. Thousands of people gathered in the rain to show their support for Russia's only national independent television station in its fight against new owners. (Photo by Oleg Nikishin/Newsmakers)

Tom Wilder, 17 from Kent, dives in the mud at the Glastonbury Festival site at Worthy Farm, Pilton on June 23, 2011 in Glastonbury, England. Music fans had to brave more rain today at the five-day festival which opened yesterday. This year the festival will feature headline acts U2, Coldplay and Beyonce. The festival, which started in 1970 when several hundred hippies paid 1 GBP to watch Marc Bolan, has grown into Europe's largest music festival attracting more than 175,000 people over five days. (Photo by Matt Cardy/Getty Images)

A wild water buffalo eats tree branches standing in flood water at the Pobitora wildlife sanctuary in Pobitora, Morigaon district, Assam, India, Thursday, July 16, 2020. Floods and landslides triggered by heavy monsoon rains have killed dozens of people in this northeastern region. The floods also inundated most of Kaziranga National Park, home to an estimated 2,500 rare one-horned rhinos. (Photo by Anupam Nath/AP Photo)

A photograph of a dog taken from underneath on December 2016 in VA, Canada. Forget the cats, itÌs just raining dogs! A creative duo have come up with an innovative way to raise money for animal welfare; photographing dogs as youÌve never seen them before. Professional photographer Jason Kenzie and designer Tania Ryan have created a unique set of images from the underside of dogs, named The Underdogs Project. The project will be featured as a 2017 calendar, with each month belonging to a quirky canine. (Photo by The Underdogs Project/Barcroft Images)

A body laying in the street is covered as police block the area around the site of a shooting in Halle an der Saale, eastern Germany, on October 9, 2019. At least two people were killed in a shooting on a street in the German city of Halle, police said, adding that the perpetrators were on the run. “Early indications show that two people were killed in Halle. Several shots were fired. The suspected perpetrators fled in a car”, said police on Twitter, urging residents in the area to stay indoors. (Photo by Sebastian Willnow/dpa/AFP Photo)

People ride a minibus illuminated with green colorful lights at a street in preparation for celebrations of the birthday of the prophet Muhammad, in Sana'a, Yemen, 04 October 2022. Mawlid is annually celebrated by many Muslim countries on the 12th day of Rabi al-Awwal (the third month of the Islamic calendar), marking the birthday of the prophet Muhammad, which is expected to be marked on 08 October 2022. In Yemen, it is celebrated by illuminating buildings and streets with green colorful lights and decorating vehicles in green. (Photo by Yahya Arhab/EPA/EFE)