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

Step inside the first house designed by Catalan architect Antoni Gaudí, considered to be one of the first Art Nouveau buildings.

The former 1920s mansion of a prominent Catalan developer was used as the Soviet Embassy during the Spanish Civil War, and there is still a bunker below the garden.

Construction of Barcelona's iconic (but controversial) church is expected to be completed in 2026—a century after the death of its architect.

The remains of the wrecked steam trawler can be found on the beach at low tide.

A grand Yugoslav memorial to the group of partisans who fought and died resisting western Serbia’s Nazi occupation.

A little-known cafe in an old copper bridge tower with 360-degree views of the harbor.

Hidden amid the tropical forest in the Caribbean Sea are the decaying ruins of the Cocaine King's luxurious island party home.

Beneath Kraków Old Town's main square lies a hologram-filled medieval market vampire graveyard wonderland.

This unassuming museum's slogan translates to “there is a limit of sadness, anxiety has no limits."

The ghost town left by the worst nuclear disaster of all time is being taken over by nature and urban explorers.

This museum promotes informed contraception by documenting its often secret past.

A Japanese cat island also populated by the descendants of a band of banished samurai.

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

This abandoned five-star hotel is now a five-star urban exploration destination.

The oldest (maybe) swimming pool in Iceland is a stunning oasis built into the side of a lush hill.

This wrecked airplane has been sitting on a black sand beach since the 1970s.

A winding, blood-red tunnel of hardened lava navigable by foot.

The most powerful waterfall in all of Europe.

Climb right into the heart of this colorful Iceland volcano.

This elegant arc of a waterfall marks a religious turning point.

Iceland's "Smoky Valley" is a beautiful river that is shrouded in a constant cover of steam clouds.

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

This volcanic cave lake is essentially hard proof that Iceland is the most magical place on Earth.