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

Austria's very first and only nuclear power station was completed but never used.

Museum devoted to the artificial language of Esperanto.

Giant WWII fortresses that fired 8,000 rounds a minute, now home to thousands of pigeons.

The world's oldest known salt mine.

This series of underground chambers contains Europe's largest subterranean lake that is slowly drained each day.

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

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

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

One of the best collections of wax anatomical models in the world.

A centuries old castle atop a dormant volcano houses museums devoted to both women and witches.

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

A New Zealand communications tower stands as the tallest structure in the Southern Hemisphere.

Major tourist destination known for its bubbling mud-pools and hot thermal springs.

Tubing through a cave lit with glowworms is a magical underground experience.

A king's pet, a pope's gift, an artist's inspiration, and a tragic shipwreck.

This unreal Portuguese palace looks as though it's made from a pile of different castles.

Unsurprisingly, Holland is home to one of the most magnificent flower gardens in the world.

Sweden's "good old days" painstakingly re-created in a model village.

It houses the remains of a 17th-century version of the Titanic.

From Tintin to the Smurfs, Europe's greatest graphic icons are celebrated in a grand Art Noveau gallery.

Climb inside an iron crystal magnified 165 billion times its normal size.

This fairytale castle was built as a show of power but was nearly torn down thanks to its history of torture.

This ancient Greek place of assembly and marketplace is being revealed layer by layer below the modern Athens cityscape.