With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.

The sci-fi villain is a little-known inhabitant of the U.S. capital's largest cathedral.

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

During the Industrial Revolution this “Temple of Invention” was full of intricate miniature machines and gadgets.

Fittingly, America's museum of architecture is itself a magnificently designed old building.

With a history of slaughter, feast, and famine, remnants of the pork industry decorate this area--even its church.

This bridge was constructed from the rubble of the infamous Bastille.

A series of abstract, silly sculptures represent the works of a great modern classical composer.

The spiral staircase hidden inside this giant Parisian column was likely built for Catherine de Medici's astrologer.

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.

Dedicated to the art of hunting, with juxtapositions of contemporary art.

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.

Remains of Roman amphitheater hidden in a sleepy quarter of Paris.

Twelve grave-faced marble angels guard the emperor's tomb at Les Invalides.

A lone cannonball remains lodged in the walls of the Hotel de Sens as a sober reminder of a revolution.

A Parisian exterminator's shop whose storefront displays stuffed rats hanging from traps.

Natural history museum celebrating biodiversity with a parade of taxidermy animals.

Exquisite wall sculptures carved in secret by an early quarryman in an obscure corner of the Paris Catacombs.

The curious underground history of keeping Paris clean.

A museum on the history of Paris, including a set of Napoleon's toiletries, Proust's room, and relics of the Revolution.

Medieval museum built over Roman baths that features famous tapestries of a lady and a unicorn.

This museum houses an imposing skeleton army comprised of creatures great and small.

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

A Parisian cemetery where the elite and artistic are interred beneath an eccentric collection of tombs.