A World War II evacuation tunnel turned into a history display of Croatian state and people.

This former railway facility is the oldest in Slovenia, and now houses a museum showcasing the history of steam locomotives.

One of the few remaining casualties of the London Blitz, this destroyed church has become an enchanting public garden.

An abandoned saucer-shaped monument in the Bulgarian mountains.

An open-air sewer system that was covered piece-by-piece over 300 years.

The vast chamber inside this abandoned cooling tower looks like something out of a sci-fi dystopia.

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

To literally keep their waste out of their own backyards, the citizens of this English city built an impressive sewer system that is still in use today.

This Slovakian statue of an emerging sewer worker might just be a bronze peeping tom.

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

An ossuary built in 1210 to collect bones from the Ospedale Maggiore of Milan.

Artistic tombs, monuments, and a beautiful "Hall of Fame" lurk inside this cemetery.

Saint Borromeo often prayed in this austere crypt during the 16th century.

This recently restored crypt was once plagued by liquefying coffins and exploding caskets.