Underground

A 1,500-year-old, 3-story subterranean city where ancient Persians took refuge in times of war.

The medieval network of canals hidden underneath the city is opened to explorers each year.

A network of hundreds of interconnected wineries in caves hides under the town center.

A mesmerizing landscape of 18th-century cellars carved out of the land.

Vienna's underworld of tunnels and subterranean rivers made an iconic appearance in a 1949 Orson Welles film.

The world's largest prehistoric copper mine had 30,000 animal bones scattered throughout the maze of passageways.

The construction of a tram line unveiled hidden remnants of the old medieval city.

These ornate manhole covers were used for a now-forgotten purpose.

A cavernous three-story, 43,800-square-foot basement that was forgotten about for 60 years.

A portal into the bricked up canal that runs through the heart of Washington D.C.

WWII-era bunkers are hidden under the beaches of this picturesque park, which is still scattered with wartime watchtowers. .

A Texas cave network is home to some of the world's most spectacular-looking and bizarrely named mineral formations.

The site where a nuclear bomb was lowered over 8,000 feet underground and detonated as an experiment in natural gas extraction.

The "Greatest Sewer" of ancient Rome is one of the oldest sewer systems in the world, and is still in use.

This network of tunnels housed an entire village for years, and even had a hall for screening movies.

A 19th-century subterranean reservoir repurposed as a contemporary art cave.

An elegant chandelier hangs over this ancient sewer chamber, now open for concerts and public tours.