People walk behind the Medusa head in the Basilica Cistern in Istanbul, Turkey, 24 July 2022. The site has been restored by the Istanbul Metropolitan Municipality and is now open for visitors again. The Byzantine structure was commissioned by Emperor Justinian and built in 532. The underground Basilica, also called Underground Cistern, is the largest well preserved cistern in Istanbul, which rests on a total of 336 columns. According to historical texts, more than 7,000 slaves were involved in the construction of the cistern. (Photo by Sedat Suna/EPA/EFE)

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)

The Lotus Exhibition Center is an icon for the city of Wujin. Conceived as an addition to an existing underground building with a lake top, the center is a sculptural and ethereal lotus flower displaying the three stages of the lotus bloom life.

Beneath the Swedish capital lies an intricate web of underground train lines. More than 90 of the 100 stations in the 110km tunnel system, sometimes referred to as “the world’s longest art gallery”, have been decorated with paintings, installations, mosaics and sculptures by 150 artists since the 1950s. After spending a couple of weeks exploring arctic Norway and Sweden, London-based travel photographer Conor MacNeill headed underground to capture images of the metro stations. Here: A rainbow arcs over a girl on the platform of Stadion station. (Photo by Conor MacNeill/The Observer)

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)

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)