Cave on extinct volcano, once home to famous mathematician Pythagoras.

The hallucinogenic center of the ancient world.

This building holds the embalmed remains of the leader of the Bolshevik Revolution.

This historic black and white monastery is one of Bulgaria's architectural jewels.

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

A temple to modernity, tucked away amid Barcelona's more well known museums.

Gaudí's famously flamboyant architectural style is on full display in this sprawling park and garden.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

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

The "dragon babies" of southeastern Europe.

A miniature train-ride through Slovenia's largest cave system.

This one-time health resort was used by aristocrats around the world.

Imperial palace of Emperor Galerius, the Emperor behind the worst Christian persecutions during the Roman Empire.

Stone age urban planning.

The bizarre rocks of Đavolja Varoš are the result of natural erosion.

Religious perseverance and cultural coexistence manifest in an architectural gem that multiple wars couldn't topple.

Destroyed Serbian army headquarters sits half crumbled in a largely revitalized Belgrade.

Nikola Tesla's ashes are held in a golden sphere, surrounded by his works.

Structure built from the skulls of vanquished foes.

The former Nazi concentration camp stands as a museum to remember this dark chapter in European history.

An underground city of salt.

The world's first and only escalator for lazy bicycles and their daunted riders.

The design of this beautifully crafted building was inspired by Norwegian glaciers.

A favorite with sightseers, this Norwegian funicular provides easy access to a stunning view of Bergen and its environs.