This Luxembourg cave is covered in cryptic looking marks left by mining activity.

This noble mash-up in eastern Luxembourg is part Medieval, part Renaissance, all castle.

Once the local rulers had gone these enchanting ruins housed common folk in an almost real life fairy tale.

This facility has manufactured stunning tapestries for France's royalty ever since the 17th century.

Underneath a chalk hill in France lies what Hitler hoped would be the underground launching base for a devastating new weapon.

The Church of the Annunciation was carved into the face of a cliff, just like the rest of the town of Haute-Isle.

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

The sunken structure looks like the victim of a catastrophic flood.

Beggars were miraculously "cured" of their fake ailments when they returned home to this 17th century Paris slum each night.

Despite being considered a thoroughly American sport, basketball's oldest surviving playing field is found in the basement of a Paris YMCA.

This ancient Roman bridge and aqueduct still stands as a wonder of classical construction.

Two small chapels housed inside an ancient tree.

The largest underground church in Europe.

Historic medieval fortified city in southwest France.

The industrial architecture honors the laborers who brought the Exposition Universelle of 1900 to life.

The oldest and smallest cemetery in Paris is only open to the public twice a year.

Barcelona's only remaining tram system takes riders to the Tibidabo funicular.

Turntable anonymously received orphans, alms, and groceries.

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

An architectural marvel that spans the Arda River, and has stories of a haunted past.

A subterranean shelter remains intact under a busy Paris railway station.

A subterranean house of worship carved right out of the bedrock.

The world's longest outdoor escalator takes visitors to the top of China's largest dam and is also shaped like a dragon.

Stockholm's last remaining paternoster elevator.