Ngorongoro Crater (Tanzania). At 610m deep and 260 sq km, this is the largest unflooded caldera in the world. A blue-green vision from above it's a haven for engangered wildlife and Maasai livestock. The crater was formed three million years ago when a giant volcano, which could have been as high as Kilimanjaro, exploded and collapsed. The caldera formed the concentric fractures in the crust cracked down to a magma reservoir deep underground. (Photo by John Bryant/Getty Images)

Handout picture released by Telam showing the tarmac of the Pedro Palacios Avenue in Ramos Mejia, province of Buenos Aires, Argentina, under which the channeled waters of the Maldonado stream flow, after it collapsed on April 8, 2014 leaving a 40-metre-long, 1.4-metre-deep hole. About 3000 people had to be evacuated in the Argentine provinces of Neuquen (soutwest), Catamarca (northwest), Santiago del estero (center) and Cordoba (center)due to the foods caused by heavy rain, official sources said. (Photo by Daniel Dabove/AFP Photo)

Raissa Floriano whose father is hospitalized with CCOVID-19, cries during a protest amid the new coronavirus pandemic outside the 28 Agosto Hospital, in Manaus, Brazil, Thursday, January 14, 2021. Scores of COVID-19 patients in the Amazon rainforest's biggest city will be transferred out of state as the local health system collapses and dwindling stocks of oxygen tanks begin to falter. (Photo by Edmar Barros/AP Photos)

Rescuers and mother surround Adnan Mohammet Korkut after he was rescued in Gaziantep, southern Turkey, early Friday, February 10, 2023. The teenager was pulled largely unscathed from beneath the rubble of a collapsed building in the Turkish city of Gaziantep early Friday, in a dramatic rescue that belied the reality that the chances of finding many more survivors four days after a catastrophic earthquake killed tens of thousands are shrinking fast. (Photo by IHA via AP Photo)

Photographer Alexander Lepeshkin spends whole nights at Clumba parties, or costume parties where the young creatives of Russia put on their most flashy and drink. His photos capture candid moments of alcohol-enhanced connections and collapses, but he says he’s not a party photographer – he’s a youth photographer. Photo: A Loshadka party in St. Petersburg, 2010. (Photo by Alexander Lepeshkin)

The annual Victory Day military parade takes place at Red Square on May 09, 2008 in Moscow, Russia. Russia's most important national holiday honours over 26 million Soviet soldiers killed during World War II. Around 8,000 soldiers in newly designed uniforms paraded in the largest Victory Day display of heavy weaponry since the collapse of the Soviet Union. (Photo by Dima Korotayev/Epsilon/Getty Images)

Officers react at their graduation ceremony during Iraqi Army Day anniversary celebration in Baghdad January 6, 2015. Iraqi Defence Minister Khaled al-Obeidi said on Tuesday that the Iraqi military has started rebuilding after its near total collapse last summer but that the effort is still in its initial phase. (Photo by Ahmad Mousa/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.