The walls and ceiling of this Polish church are decorated with thousands of skulls, with another 21,000 skeletons just below.

Houses are tucked between, on, and underneath giant boulders.

A 16th century chapel decorated with bones, skulls, and entire bodies hanging from the wall.

One of the most beautiful bookstores in the world hides a neo-Gothic interior behind an art nouveau facade.

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

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

A submerged village appears when dam levels drop.

This memorial park stands on the site of one of the largest mass executions of the Second World War.

For over 40 years, this quaint little house has teetered on a rock in the middle of a river.

This 14th century castle withstood years of battle but is now sliding off the hills into the Danube.

A perfectly preserved hill village where the buildings are painted like white lace.

The blood-soaked ruins of the castle where the murderous Blood Countess was walled in alive.

A museum dedicated to the Spanish occult in a town that was terrorized by the Inquisition.

Three bridges and one giant canyon make up this fantastic Spanish settlement.

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

Sweden's first nuclear reactor, R1, can be visited 25 meters underground.

Bring your hiking shoes, wool socks, and thermal underwear for a stay in these cozy forest huts.

The world's first ice hotel is still one of the most elaborate and beautiful.

The world's largest model of our planetary system.

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

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

This memorial commemorating fallen Swiss Guards of the French Revolution has been called the saddest stone in the world.

This amazing pedestrian suspension bridge in the Swiss Alps would give Indiana Jones nightmares.

Caves famous for the dragon-slaying saint who lived there.