Turkey's Derinkuyu Underground City was built over 1,800 years and can accommodate over 20,000 people, including livestock and stores. Getty Images
Going underground: City Hall subway station, New York. The tiled, vaulted ceilings look down on the platform which was last used on December 31, 1945. Getty Images
The chandelier-filled cathedral carved out of the rock salt in Wieliczka salt mine, Poland, which has been on the Unesco World Heritage List since 1978. Supplied
Religious artwork carved into the walls of the Wieliczka salt mine, including Leonardo da Vinci's 'The Last Supper'. Getty Images
Among the tunnels and chambers of the mine are many statues and sculptures carved out of salt. Getty Images
Statue of a miner sculpted from a single piece of rock salt. Getty Images
Work on the underground Edinburgh Vaults was completed in 1788 and visitors can tour the many abandoned taverns and workshops. Getty Images
Part of the Pilsen Historical Underground tunnels in the Czech Republic, which started construction in the 13th century. Alamy
The Pilsen medieval network of vaulted subterranean tunnels stretches for more than 12 miles and contains replicas of former features, such as this water wheel. Alamy
Archaeologists have found items dating back to the Middle Ages in the walls of the Pilsen Historical Underground. Alamy
Derinkuyu Underground City in Cappadocia, Turkey, dates back to the 7th and 8th centuries BC. Getty Images
Turkey's Derinkuyu Underground City was built over 1,800 years and can accommodate over 20,000 people, including livestock and stores. Getty Images
Going underground: City Hall subway station, New York. The tiled, vaulted ceilings look down on the platform which was last used on December 31, 1945. Getty Images
The chandelier-filled cathedral carved out of the rock salt in Wieliczka salt mine, Poland, which has been on the Unesco World Heritage List since 1978. Supplied
Religious artwork carved into the walls of the Wieliczka salt mine, including Leonardo da Vinci's 'The Last Supper'. Getty Images
Among the tunnels and chambers of the mine are many statues and sculptures carved out of salt. Getty Images
Statue of a miner sculpted from a single piece of rock salt. Getty Images
Work on the underground Edinburgh Vaults was completed in 1788 and visitors can tour the many abandoned taverns and workshops. Getty Images
Part of the Pilsen Historical Underground tunnels in the Czech Republic, which started construction in the 13th century. Alamy
The Pilsen medieval network of vaulted subterranean tunnels stretches for more than 12 miles and contains replicas of former features, such as this water wheel. Alamy
Archaeologists have found items dating back to the Middle Ages in the walls of the Pilsen Historical Underground. Alamy
Derinkuyu Underground City in Cappadocia, Turkey, dates back to the 7th and 8th centuries BC. Getty Images
Turkey's Derinkuyu Underground City was built over 1,800 years and can accommodate over 20,000 people, including livestock and stores. Getty Images
