The perfectly preserved underground rooms where Churchill plotted the war against Germany.

This South London cemetery holds the remains of more than 15,000 people.

An easily overlooked stretch of ornate window fronts and gaslight lamps that could be right from the pages of Dickens.

This collection of illusory paraphernalia has been culled from the history of London's premier magician's club.

This shop and gallery is a modern-day cabinet of curiosities, unassumingly positioned along an urban thoroughfare.

A kaleidoscopic warehouse-maze of handmade neon signs.

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.

London's creepiest cemetery was once the site of dueling magicians and mobs of stake-carrying vampire hunters.

A former steam-powered mill from the 1800s is now an industrial park with a giant steel dragon slide.

The unexpectedly beloved remnants of a controversial thermal power plant.

For one day every month Barcelona's Estació de França transforms into a market for model trains.

The former 1920s mansion of a prominent Catalan developer was used as the Soviet Embassy during the Spanish Civil War, and there is still a bunker below the garden.

Turntable anonymously received orphans, alms, and groceries.

A stately palace has been converted into the world's largest cannabis museum.

These secluded and abandoned anti-aircraft defenses have become one the most scenic viewpoints in Barcelona.

Mountain where Jesus was first tempted by the Devil.

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

A beautiful but toxic fountain of mercury.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

The oldest volcanology research center in the world.

For decades, vendors have sold clothes once worn by American soldiers who stormed Italy in the 1940s.

The meaning behind this triumphal Roman arch was a mystery until the early 20th century.

A small cell adjacent to this church was once the bedroom for a renowned saint.

This medieval city gate is now a blend of the past and the modern world.