A worker stands inside the Cuncas II tunnel that will link canals being built to divert water from the Sao Francisco river for use in four drought-plagued states in Brazil, near the city of Mauriti, Ceara state, on January 28, 2014. #

A member of the public is winched to the surface after visiting the Main Chamber of Gaping Gill, the largest underground cavern in Britain naturally open to the surface, near Ingleton, England, on May 26, 2015. At a depth of almost 100 meters from the surface, trips into Gaping Gill cavern are offered twice a year to members of the public alternatively by Bradford Pothole Club and Craven Pothole Club. #

A tourist views the enormous pressure-adjusting underground water tank during a tour of the Metropolitan Area Outer Underground Discharge Channel facility on April 8, 2015, in Tokyo, Japan. The Metropolitan Area Outer Underground Discharge Channel is the world's largest underground flood diversion facility. The facility takes water overflow from four rivers and redirects it 50 meters underground into a 6.3 kilometer tunnel before being pumped into the Edo river. Construction of the facility began in 1993 and became fully operational 13 years later in 2006. The facility, designed to drain flood waters from the surrounding areas after heavy storms or typhoons is used on average seven times a year and has also become a tourist attraction. #

Nihal, 9, looks out the entrance of an underground Roman tomb used as shelter from Syrian government forces shelling and airstrikes, at Jabal al-Zaweya, in Idlib province, Syria, on February 28, 2013. Across northern Syria, rebels, soldiers, and civilians are making use of the country's wealth of ancient and medieval antiquities to protect themselves from Syria's ongoing war. They are built of thick stone that has already withstood centuries, and are often located in strategic locations overlooking towns and roads. #

A visitor uses a staircase at the British Museum's new World Conservation and Exhibitions Center on July 11, 2014, in London, England. The new £135 million building extends 20 meters underground and contains new storage and conservation spaces. #

A young miner leads his team of donkeys back to the coal face underground in Choa Saidan Shah in Punjab province, Pakistan, on April 29, 2014. Coal miners in Chao Saidan Shah use donkeys to transport coal from the depths of the mines to the surface. The donkeys make around 20 trips per day carrying sacks weighing about 20 kg (44 lbs) each. The work is dangerous with the constant risk of cave-ins. The miners say they do what they can to care for the animals, with their limited resources, but the difficult conditions mean the donkeys' life expectancy is 12-13 years. #

Rebel fighters dig caves in the mountains, to be used as shelters from the bombing of forces loyal to Syria's President Bashar al-Assad, in al-Latamna town, in the northern countryside of Hama, on March 9, 2015. #

The Large Hadron Collider (LHC), photographed during a media visit at the Organization for Nuclear Research (CERN) in the French village of Saint-Genis-Pouilly near Geneva, Switzerland, on July 23, 2014. #

Homes in the village of Kandovan, 650 km (400 miles) northwest of Tehran, Iran, on June 26, 2007. Kandovan is a village where homes are dug out of the rock formations in the foothills of the Sahand mountain. #

A worker climbs the exit stairs in the disused Down Street underground station on May 18, 2015, in London, England. The disused underground railway station, which closed in 1932, was used by wartime leader Winston Churchill for some cabinet meetings as his Whitehall bunker was being built. Transport for London are seeking ideas from businesses that may want to lease some of the 400 square meters available at the station. Out of 270 functioning stations in London there are at least 40 overground and underground stations which still exist but are no longer used. #

An archaeologist digs out skeletons from the site of the graveyard of the Bethlehem, or Bedlam, hospital next to Liverpool Street Station in the City of London August 7, 2013. The dig is on the site of the future ticket hall for the Crossrail station at Liverpool Street. Work began in 2009 and digging the new tunnels has uncovered a range of archaeological finds including 20 Roman skulls and a graveyard which could hold the remains of some 50,000 people killed by the plague more than 650 years ago. #

The cutting head of "Bertha," the massive tunnel boring machine currently stopped underground near downtown Seattle while waiting for repairs, at the bottom of a 120-foot access pit after a 270-ton section of the front shield was removed and lifted to the surface with a crane on March 19, 2015. The piece is the first of several big crane lifts that will be needed to bring the cutting head and other parts to the surface to complete repairs on the machine, which was digging a 1.7-mile highway tunnel as a replacement for the Alaskan Way Viaduct. #

Enoch Foster, a fundamentalist Mormon practicing polygamy, along with his first wife Catrina Foster and several of his 13 children from his two wives, enter the Charity House at the Rockland Ranch community outside Moab, Utah, on November 2, 2012. The "Rock" as it is referred to by the approximately 100 people living there in about 15 families, was founded about 35 years ago on a sandstone formation near Canyonlands National Park. Polygamy was a part of the teachings of The Church of Jesus Christ of Latter-day Saints and was brought to Utah by faithful Mormons in the late 1840s. The mainstream Mormon church abandoned the practice in 1890, but an estimated 37,000 Mormon fundamentalists continue the practice today and believe plural marriage brings exaltation in heaven. #

Displaced Yemenis stand in a man-made underground water tunnel where they are taking shelter after their houses were destroyed by air strikes carried out by the Saudi-led alliance, in the Yemeni capital, Sanaa, on April 29, 2015. #

Members of the public visit the Main Chamber of Gaping Gill, the largest underground cavern in Britain naturally open to the surface, near Ingleton, England on May 26, 2015. #

Tourists on traditional boats sail in the Beihai military water tunnels on the island of Nangan in the Matsu archipelago, off northern Taiwan, on May 8, 2013. The Matsu archipelago, which was once front line against China, is now a military tourist spot. #

Pamela Knox waits for rescue after a massive sinkhole opened up underneath her car in Toledo, Ohio in this July 3, 2013 photo. Toledo firefighters later rescued Knox without major injuries. Fire officials told a local TV station that a water main break caused the large hole. #

Hindu devotees gather at the shrine in Batu Caves temple during Thaipusam in Kuala Lumpur on February 3, 2015. Hindu devotees across Malaysia were celebrating Thaipusam, a religious celebration dedicated to the Hindu deity Lord Murugan. Piercing various parts of their body with silver skewers, devotees carry often large contraptions known as Kavadi, thereby taking on a physical burden through which the devotees implore for help from Murugan. Devotees also fulfill vows by carrying milk-filled pots up the stairs to the cave temple. #

A wildcat miner drills stones containing gold materials in an informal tunnel near Relave, a mining village at the province of Parinacochas in Ayacucho, Peru, on February 20, 2014. Tucked between two desert ridges in southern Peru, Relave looks like any of the thousands of ramshackle mining towns that blight the landscape in the world's sixth-largest gold exporter. But Relave is also home to Aurelsa, one of the first small-scale mines in the world to produce gold certified and marketed as "ethical" as part of a scheme aimed at reducing the harmful impact of illegal mining in mineral-rich developing countries. #

A tourist visits the Columbarium caves located at the Beit Guvrin-Maresha National Park in central Israel, on June 24 2014. The caves of Beit Guvrin-Maresha are a network of hundreds of man-made caves, excavated near the ancient towns of Maresha and Beit-Guvrin, that bear witness to a succession of historical periods of excavation and usage stretching over 2,000 years, from the Iron Age to the Crusades, as well as a great variety of subterranean construction methods. #

View of a saltwater pond at Nemocon's salt mine in Nemocon, Cundinamarca, Colombia, on November 22, 2012. The 80-meter-deep mine has over 500 years of history, and has become a popular tourist destination in Colombia. #

Children play in the salt mine at the state republican clinic of speleotherapy near the town of Soligorsk, south of Minsk, on February 19, 2015. More than seven thousand children and adults in a year treat diseases of the respiratory tract in the subsurface chambers, which are located 420 meters underground in the layers of potassium and stone salts in the salt mine. #

Francisca Gomez poses for a photograph inside her home, a cave where she has been living for the past 50 years, in the mountains on the outskirts of Chusmuy, Honduras, on April 21, 2015. The 75-year old widow receives help from neighbors and friends and she makes a little money by selling wood she collects, according to a local newspaper. #

A Palestinian fighter from the Izz el-Deen al-Qassam Brigades, the armed wing of the Hamas movement, stands inside an underground tunnel in Gaza on August 18, 2014. A rare tour that Hamas granted to a Reuters reporter, photographer and cameraman appeared to be an attempt to dispute Israel's claim that it had demolished all of the Islamist group's border infiltration tunnels in the Gaza war. #

The entrance of a tunnel, dug by thieves in order to enter the safe deposit room of a bank in Berlin, photographed on January 17, 2013. Robbers dug the 100-foot (30 meter) long passage, which was discovered by police on Monday, from a nearby underground parking garage into a branch of Berliner Volksbank and stole the contents of several hundred safe-deposit boxes. #

Helmets and gas maskssit inside a secret bunker of Fascist leader Benito Mussolini that was built between 1942 and 1943 under his private residence at Villa Torlonia in Rome, on October 25, 2014. #

Cars from 1974 sit abandoned in an underground garage in the United Nations (U.N.) buffer zone in central Nicosia, Cyprus , on March 12, 2014. The cars were imported from Japan and intended to be sold at a Toyota dealership. They were stored here for safety during the 1974 conflict but have not been moved since. Some cars have only 32 miles (52 km) on the odometer. #