Passengers pose for a selfie as they wait for the Night Tube train service at Oxford Circus on the London underground system in London, Britain August 20, 2016. The London Underground is starting its first-ever overnight service, a move city leaders hope will make the British capital a truly 24-hour city and bolster the local economy. (Photo by Paul Hackett/Reuters)

Female labourers wearing helmets take a break from laying underground electricity cables in Ahmedabad, India, March 7, 2016. (Photo by Amit Dave/Reuters)

Tami Varma, right, and her brother Robin, the grandchildren of Devendra Varma, a scholar of English gothic tales and an expert in vampire lore, pose in coffins at the Bran Castle, in Bran, Romania, Monday, October 31, 2016. A Canadian brother and sister are passing Halloween night curled up in red velvet coffins in the Transylvanian castle that inspired the Dracula legend, the first time in 70 years anyone has spent the night in the gothic fortress, after they bested 88,000 people who entered a competition hosted by Airbnb to get the chance to dine and sleep at the castle in Romania. A portrait of medieval prince Vlad the Impaler is placed on the wall. (Photo by Vadim Ghirda/AP Photo)

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.

Taken by photographer, Denis Budkov, 33, the Plosky Tolbachik is one of a cluster of volcanoes located in the Kamchatka Peninsula in the far east of Russia. After taking a lucky group of tourists to the crater of the flat volcano, the Russian photographer managed to get less than 300 metres away from the spurting jets of lava. Photo: The underground entrance to the volcano resembles a scene from the latest Hobbit movie. (Photo by Denis Budkov/Caters News)

The series of shots show the bare steel infrastructure of the Bushfield army training camp near Winchester which was in operation during World War Two and was used to train Royal Green Jackets recruits in the sixties. The spectacular images were taken by an urban explorer who wished to remain anonymous. Here: A railway siding in Wiltshire, UK where ammunition was transferred by tunnel to an underground storage facility. (Photo by MediaDrumWorld.com)

Sam Warren climbs in to a prototype combat vehicle during the PrepperCon expo Friday, April 24, 2015, in Sandy, Utah. Hundreds of survivalists and “preppers” are gathering in the Salt Lake City suburb of Sandy for the first annual PrepperCon, featuring demonstrations of underground bunkers, food storage, and armored vehicles. (Photo by Rick Bowmer/AP Photo)