A limestone wonderland of water and subterranean secrets.

A perfectly circular stronghold constructed in the 10th century is now a nucleus of Viking culture housing a museum, village, and reenactment of an epic Viking battle.

Vortex-shaped aquarium hosts Europe's largest school of piranhas and other ocean creatures.

Six beautiful wooden giants are hidden around Copenhagen.

The largest ice cave in the world is thought to be the entrance to hell according to Austrian legend.

The beautiful, ornate weather station is the most popular geocache in Salzburg.

Amidst medieval and baroque buildings, five human-sized gherkins stand tall.

The world's oldest known salt mine.

An Austrian house of bones filled with hundreds of intricately painted skulls.

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

The fairy tale castle of the "fairy-tale king."

An eerie, derelict amusement park in Berlin.

This jaw-dropping 19th-century bridge uses its reflection to form what appears to be a perfect circle.

The house that took a plunge into the facade of a museum.

This spire on an Austrian mountain is wrapped around a long spiral slide and has the world's tallest wooden observation deck.

Visitors dip into a variety of pools of what was once ice.

Each year melting snow turns these Austrian hiking trails into scuba zones.

The former private quarters and office of Dr. Sigmund Freud now hosts a museum housing the works of the founding father of psychoanalysis.

Swarovski's museum/theme park is a fake diamond fantasia overlooked by a giant.

The topiary maze at the Habsburg hunting lodge.

Where beer lovers can literally immerse themselves in beer.