A young girl runs through a section of an underground city on April 17, 2016, in Nevsehir, Turkey. Cappadocia, a historical region in Central Anatolia dating back to 3000 B.C is one of the most famous tourist sites in Turkey. Listed as a World Heritage Site in 1985, and known for its unique volcanic landscape, fairy chimneys, large network of underground dwellings and some of the best hot air ballooning in the world. #

A cave hotel is seen at sunset in the town of Goreme on April 17, 2016 in Nevsehir, Turkey. Despite Turkey's tourism downturn due to the recent terrorist attacks, internal instability, and tension with Russia, local vendors expected tourism numbers to be stable mainly due to the unique activities on offer. Unlike other tourist areas in Turkey such as Antalya, which is popular with Russian tourists, Cappadocia caters to the huge Asian tourist market. #

A member of staff from the 'London Transport Museum' walks down a stairway in the Down Street underground station on April 13, 2016, in London, England. The London Transport Museum was giving tours as part of their new 'Hidden London' season 2016. Down Street station in Mayfair operated between 1907 and 1932 and after closing, played an important part during the Second World War, when it was transformed into the Railway Executive Committee's bomb proof shelter. During the height of the Blitz, British Prime Minister Winston Churchill took refuge in the station tunnels. #

A peshmerga fighter walks through the kitchen of an underground tunnel made by ISIS fighters on October 18, 2016. The Kurdish forces found the tunnel in the town of Badana that was liberated from the Islamic State group on Monday. The fighters built tunnels under residential areas so they could move without being seen from above, to avoid airstrikes. #

Volunteers with the Israeli Antique Authority work inside the Cave of the Skulls, an excavation site in the Judean Desert near the Dead Sea, Israel, on June 1, 2016. #

Jozef Jarosz stands on March 16, 2016 in an underground hideout in Stankowa, Poland, where he and his family helped Anna Grygiel and 13 other Jews to survive the Holocaust and WWII. #

A highliner walks on a slackline to beat the highline world record by walking a distance of 230 meters at 100 meters underground in a cavern inside La Verna, one of the ten largest underground halls in the world, part of the Gouffre de la Pierre-Saint-Martin in Saint-Engrace, southwestern France. #

A picture taken on June 21, 2016 in Gavet, in the French Alps, shows a turbine under construction in the new underground "Romanche Gavet" hydroelectric plant, planned to begin operating in 2020, after a three-year delay. #

Internally displaced children attend a class inside a cave in the rebel-controlled village of Tramla, in Idlib province, Syria, on March 27, 2016. A group of people who live in a cave have set up a school for children during the day. The cave accommodates around 120 students, divided into two shifts. #

A lone worker does some cement finishing work inside a massive ventilation chamber on October 3, 2016, near the pit where a tunnel boring machine known as "Bertha" will emerge when the nearly 2-mile tunnel of the Highway 99 project to replace the earthquake-damaged Alaskan Way Viaduct is completed in Seattle. Officials said that the machine had passed the halfway mark as it digs beneath the city. #

Ventilation stacks sit on top of the future control building for the nearly 2-mile tunnel of the Highway 99 project to replace the earthquake-damaged Alaskan Way Viaduct, Monday, October 3, 2016, in Seattle. #

Artists perform during a show on the opening day of the Gotthard rail tunnel at the fairground Rynaecht at the northern portal in Erstfeld, Switzerland, on June 1, 2016. The new Gotthard Base Tunnel (GBT) became the world's longest railway tunnel when it opened in June. The 57-kilometer (35.4-mile) tunnel, which runs under the Alps, was first conceived in sketch-form in 1947, but construction began 17 years ago. #

Former Soviet Army officer Leonid Konovalov enters underground bomb shelter at the abandoned former Soviet R12 nuclear missile base in Zeltini, Latvia, on July 22, 2016. #

A devotee hold his hands as he prays while standing in the cave of Sufi saint Bari Imam in Lohe Dandi on the outskirts of Islamabad, Pakistan, on July 30, 2016. Everyday, hundreds of devotees pay homage and cross a distance of 10km to climb up hills to visit cave where the Sufi saint spent time. People pray and tie colorful threads and padlocks to fulfill their wishes. #

A rescue member in full protective gear stands amongst South Korean soldiers wearing gas masks during an anti-terror drill on the sidelines of a South Korea-US joint military exercise, called Ulchi Freedom, at a subway station in Seoul on August 23, 2016. #

An Iraqi soldier inspects a tunnel as he holds a position on the frontline on April 9, 2016 in the town of Kharbardan, located 10 kilometers (6 miles) south of Qayyarah, during military operations to recapture the northern Nineveh province from ISIS jihadists. #

A view inside The Abbey Mills pumping station in Stratford on May 25, 2016 in London, England. The Grade II listed building, designed by British engineer Joseph Bazalgette to lift sewage from the low-lying areas of north London, was completed in 1868. Thames Water marked Sewage Week 2016 with a series of events inviting members of the public down into the underground sewer network and around the Abbey Mills pumping station in east London. #

Kurdish Peshmerga soldiers stand around a tunnel dug by ISIS in a house recently recaptured by the Kurds during the battle to retake Mosul, on October 18, 2016 in Bartella, near Mosul in Iraq. #

Workers are seen in an undergound train tunnel at the Saint Michel subway station in Paris on July 29, 2016, during the renovation operations of the RER C, one of the five rapid transit system lines of the French capital. #

Metalsmith James Gillaspie wears medieval armor he made while waiting for the subway at the Times Square station on the way to Comic Con on October 7, 2016, in New York. #

Tourists visit the Cueva de Los Verdes, a volcanic cave created as a result of the eruption of La Corona that flows into the sea,on the Spanish Canary island of El Lanzarote on March 23, 2016. #

Lights illuminate the operating theater in an underground gallery of a Spanish Civil War-era bomb shelter in Almeria on May 23, 2016. There were 67 entrances spread out across the city to the underground network of air raid shelters some nine meters below the surface that extended for 4.5 kilometers (2.8 miles) and could fit 37,000 people -- roughly three-quarters of the city's population at the time. Almeria city councilor Miguel Cazorla, pushed to have a nearly one kilometer stretch of the shelters restored and turned into a museum which opened in 2006 drawing just under 22,000 visitors in 2015. #

In this photo from April 14, 2016, Johnny Morris, founder of Bass Pro Shops, walks around the edge of a massive sinkhole that opened up last spring on his golf course in Ridgedale, Missouri. Morris, a cave enthusiast for years, believes the sinkhole and a cave he discovered about a half-mile away in 1993 are connected after water from a pond on the golf course drained into the hole and began pouring out the entrance to the cave. #

Tourists ride on a train in Postojna Cave on April 25, 2016 as their way to see olms, ancient underwater predators which can live up to 100 years and only breed once in a decade, in one of the aquaria. The strange, slithery creatures inside Slovenia's Postojna cave were once considered living proof that dragons existed, prompting locals to give it a wide berth. Now large crowds from all over the world queue up to witness the extremely rare hatching of the mysterious olms. Found primarily in Balkan cave rivers, the protected eel-like species has been living in the world-famous Postojna cave, 50 kilometers (30 miles) southwest of the capital Ljubljana, for what researchers say is millions of years. #