A rocky sea serpent hides caves, blowholes, and a spectacular natural bridge.

A part of the world's largest marsh seems to be growing bright red Martian grass.

This 1967 experiment in modular architecture was designed to be a new model for urban living.

19th century pendulum and a clock restored by a rogue group of guerilla artists.

A floating cat sanctuary on an Amsterdam canal.

Amid the bustle and noise of the Financial District hides a secluded garden oasis above the city streets.

The London statue that imitates passersby.

You'd never know the houses at 23-24 Leinster Gardens were fakes—until you see the train tracks on the other side.

The world's first underground farm is in a World War II bomb shelter beneath the streets of London.

Remnants of a Victorian garden feature sitting in public woodland.

Several plaster noses are hidden in plain sight around London's Soho neighborhood.

A student driver's nightmare.

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

One of the few remaining casualties of the London Blitz, this destroyed church has become an enchanting public garden.

What looks like a normal building is really a hollow facade hiding a subway vent.

A simple plaque on the facade is the only reminder that the treaty ending the American Revolution was signed inside.

The water-powered carriages are long gone, but for over a century this funicular railway has been the fastest route to the highest point in Paris.

135 bronze medallions mark a French meridian line which once rivaled the hotshot Greenwich timeline.

This off-limits room in the National Library of France is home to a system of pneumatic tubes, once used to bring books to readers.

A modern-day Lascaux cave underneath Paris.

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

The first green space constructed on an elevated viaduct.

An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.

The ruins of a Duke's bucolic dream, where camels once roamed alongside "Dutch" windmills and faux-Italian vineyards.