A charming Spanish village sits atop this natural limestone overpass.

Stunning Roman aqueduct used for 1900 years and still standing.

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

Hundreds of stone stairs and a winding medieval bridge connect this haunting Spanish island to the mainland.

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

A piece of the front end of a British cruiser that was used as a block ship in World War I.

This former coal mine, now a fascinating museum, was the site of Belgium's worst mining disaster.

The last Oostende trawler to fish off Iceland is now a museum.

Visitors to this Belgium graveyard are greeted by the grieving stone parents of the young men lost to war.

The world's tallest boat lift stands as a massive piece of industrial tourism.

An early 20th-century sanatorium, long abandoned and now being reclaimed by nature.

This Belgian palace was long the domain of priests and aristocrats but is now being turned into an art commune.

This memorial museum chronicles the apocalyptic destruction of the First World War.

This charming, hidden guild museum holds some 700 years of crossbow history.

This gruesome biblical parade celebrates the one day a year that a bit of Jesus' blood turns to liquid.

What was once a bustling mining installation is now a bustling hub of contemporary art.

Head underground and take a tour of the city's working sewer system, and see the underground river Senne, which defined Brussels for centuries before being covered in the 19th century.

Only museum in the world devoted to bookbinding and book arts holds a permanent collection representing five centuries.

Housing the groundbreaking maps and globes of Gerard Mercator.

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

A Cold War command center is hidden under an idyllic countryside.

This bike path traverses the treetops.

Descend into the tunnels to experience the life of a coal miner.

Rare wooden escalators lead deep down into an otherwise staggeringly monotonous tiled tunnel.