Micro-nation based around a nine-story high illegally built tower.

A hidden bathhouse in one of Stockholm's oldest and busiest neighborhoods is open to anyone who can find it.

A floating underwater hotel, created by a Swedish artist.

The world's largest model of our planetary system.

From Coke II to Google Glass, this exhibit features some of the most spectacular commercial flops.

A series of modernist treehouse hotel rooms grace the outskirts of an Arctic forest.

Stockholm's decidedly unique public transit system has doubled as the world's longest art gallery for decades.

Wild lights and sound effects turn a walk through some caves into an eye-popping 1960s tourist attraction.

This vast, water-filled underground labyrinth can only be toured by boat.

A serpentine highway twists and turns over the treacherous Norwegian Sea.

This giant boulder is pinched over a 1000-meter abyss by two mountains.

St George's Leprosy Museum depicts the gruesome afflictions of those suffering from the disease.

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

A tour of underground Berlin takes you through WWII air raid shelters and tunnels used to smuggle people out of East Germany.

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

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

Home to the deepest rift anywhere on Earth.

A 19th-century subterranean reservoir repurposed as a contemporary art cave.

Creek water that starts here has a 50/50 chance of ending up in the Atlantic or the Pacific.

These fantastically named natural pools look like something out of a Scottish myth.

This disappearing geyser has been attributed to witchcraft, but really it's all about rainfall.

This fantastical waterfall is hidden in a cave located in a cracked cliff.

Giant, natural amphitheaters made of delicate geological formations called "hoodoos."

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