Once the home of a hermetic monk, this cave now houses a small church.

An underground city of salt.

Roman ruins top white travertine terraces formed by ancient hot springs.

This massive collection of British military artifacts includes the heartbreaking personal letters of the men in the trenches of WWI.

On the Spree bank, discover one of the last reminders of a symbolic wall that divided, and was torn down by the people.

A series of caves under Castle Hill that once held the prisoner Vlad Tepes, better known as Count Dracula.

This statue was given to France by the United States to commemorate the centennial of the French Revolution.

The last moments of the ill-fated Pompeiians, frozen forever in plaster.

The only "Egyptian" pyramid in Europe and the legendary tomb of Remus.

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

Thousands of coffin-like pillars make up this controversial Holocaust memorial.

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

One of the oldest scientific gardens in Britain, containing more than 13,302 plant species in its core collection.

Stalactites and stalagmites in a beautifully lit cave.

A master of trompe-l'oeil creates a stunning, tourist-fooling dome out of a flat ceiling.

High atop the Eiffel Tower is a small apartment built exclusively to entertain the science elite and make the rest of Paris jealous.

Cat art as far as the eye can see.

The largest free-standing aquarium in the world resides in a hotel lobby in Berlin.

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.

Northern Ireland's Brobdingnagian stepping stones.

Macabre astrological automaton clock dating to the late 1400s.

Famously misunderstood wonder of the ancient world

The only surviving piece of Copenhagen's 16th-century defensive wall.