Underground

A massive cave complex boasts some of the clearest water (and most charming bats) in the world.

This famous Nevada gold mine, once a hotbed of Wild West shenanigans, is now open for tours.

An abandoned Nazi explosives plant founded by the inventor of dynamite is now an underground museum.

An expansive complex of subterranean tunnels is all that remains of one of Europe's most formidable medieval fortresses.

A centuries-old complex of hand-dug stone chambers has sheltered locals in times of trouble.

The first tunnel to be dug beneath a river, repurposed as an art and museum space.

This luxury hotel suite is located deep in the bowels of an old mine.

This hidden stream beneath Indianapolis makes an appearance in the best-selling novel "Underground Airlines."

The Church of the Annunciation was carved into the face of a cliff, just like the rest of the town of Haute-Isle.

Underground tunnels once home to crime are now back open to the public.

Nearly 40 miles and 1,000 years of limestone mines under Denmark.

Barcelona's historic 19th-century sewers, whose foundations were laid in medieval times, are open to the public for exploring.

A 90-year-old underground water reservoir offers a more industrial kind of tranquility.