The largest of London's subterranean rivers and once a mad, bad center of London life.

A 300-year-old tea shop that brought tea to the English people, not to mention the Queen herself.

The studio of Catalonia's most famous theatrical designer remains exactly as he left it.

This former Baroque palace on the historic Calle Montcada is a rare architectural gem and was once the meeting place of an aristocratic scholarly society.

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

This cemetery features modernist statues as strange as its name.

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

Behind the pink curtain, Au Bonheur du Jour, an art gallery of erotic archives.

Noirish green neon marks the location of one of the oldest private detective agencies in France.

This Paris art space celebrates the creativity and ingenuity that springs from abject poverty, especially in Africa.

Down a cobblestone alleyway is a museum dedicated to Romanticism, with a secret garden.

A favorite Paris bar for U.S. expats, including Gershwin, Hemingway, and James Bond.

A Belle Epoque bordello turned into a themed love hotel.

Egyptian Revival movie palace abandoned for decades and restored in 2013.

A wunderkammer of mysterious objects and stories.

The subterranean "lake" below the Paris opera house inspired the Phantom of the Opera's lair.

A museum of carnival attractions where you are invited to ride the rides.

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

France's most famous cemetery, with some of its most curious tombs.

The oldest stone house in Paris was built by its most famous alchemist.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

Notorious artist squat renovated into legal studios.

Home of the future Emperors of Russia.