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. At a depth of almost 100 meters from the surface, and with a volume comparable to the nave of York Minister, 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 train passes through a tunnel in the rocks as people walk the foot-path “El Caminito del Rey,” or King's little path, a narrow walkway hanging and carved on the steep walls of a defile in Ardales near Malaga, Spain, on March 15, 2015. #

A construction crane slowly lowers a segment of the tunnel drilling machine ‘Selina’ into the circular excavation pit to depth of 24 meters at a construction site for a new tunnel underneath the river Main in Frankfurt, Germany, on February 4, 2016. #

Skulls and bones on display in the Fontanelle cemetery in Naples, southern Italy, on November 2, 2015. The cemetery is the epicenter of what is known as ‘The Neapolitan Cult of the Dead,’ or ‘The Neapolitan Skull Cult.’ It is a vast underground ossuary located in a cave in the tuff hillside at the heart of the Sanita quarter, once used to bury the corpses of people for whom there was no room in the public graves at the churches within the city. #

People take a stroll on the floor of an underground cistern, used to hold water from rivers around the facilities temporarily to avoid floods in case of heavy rain, in Kasukabe, Saitama prefecture, a suburb of Tokyo, Japan, on November 14, 2015. The Land Ministry opens the cistern, which is 14,000-square-meters and 18-meters-high, to the public once a year where several thousand people visit the so-called “the underground temple.” #

Vistors swim in the To Sua Ocean Trench on September 12, 2015, in Lotofaga, Samoa. The To Sua Ocean Trench with a literal translation in English of "large swimming hole" is a 30 meter deep hole connected via underground canals to the ocean, located in the Lotofaga village on the south coast of Upolu island. #

Overhead lights illuminate one of several recently discovered ancient tunnels underneath the city of Puebla, Mexico, on September 3, 2015. Puebla is one of the oldest cities in Mexico, founded around 1531. The city is excavating old tunnels and waterworks under the city’s downtown area, with a view to possibly opening them as a tourist or cultural attraction. #

The main conference room inside a nuclear bunker site on the Woodside Road industrial estate in Ballymena, Northern Ireland, on February 4, 2016. The underground shelter has been put up for sale by the offices of the Northern Ireland First and Deputy First Minister. The bunker which was completed in 1990 was built to hold up to 235 people in the event of a nuclear bomb and is complete with kitchen facilities, dormitories, and decontamination chambers. #

Visitors to the Lowline Lab inspect the plants growing under a solar canopy on February 27, 2016, in New York. Optical devices capture focus and distribute full spectrum light it into a solar canopy that then distributes the sunlight inside the warehouse. The Lowline Lab, an experimental phase in the creation of an underground park on the Lower East Side of Manhattan, is free and open to the public on weekend days through June 2016. #

A tunnel with tracks for mining cars, part of the Nazi Germany “Riese” construction project, pictured near an area where a Nazi train is believed to be, in Walim near Walbrzych southwestern Poland, on August 31, 2015. #

Abu Amer (center), a former well driller, sits with family members inside their underground shelter in an Aleppo's southern suburb on October 29, 2015. The shelter which Abu Amer has been building for the past year, and will include four rooms equipped with ventilation and lighting, was expected to be completed in the coming months. #

A Greek Orthodox monk prepares breakfast at Our Lady of Hamatoura Monastery in northern Lebanon on March 8, 2016. The monastery's history goes back to the fifth century. It faced destruction and persecution during the Mamluk and Ottoman eras and was restored in 1994. #

Pakistani children sit in a cave where they live with their families in Hasan Abdal, on February 4, 2016. Hundreds of of families live in caves as their homes equipped with all modern amenities, such as running water, electricity, and cable television, built by their forefathers near Hasan Abdal. #

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. #