Originally conceived as part of a proposed Arizona State Capitol building in the 1950s, this structure built in 2004 stands as a monument to the architect's legacy.

This abandoned caretaker's apartment is one of 13 left in the New York Public Library system.

The signs of a long buried road can still be found hidden beneath a London sewer grate.

Built as a gift to the hottest tastemaker of the time, these hidden quarters remain an Art Deco showpiece.

The famous facade of Grand Central Terminal hides a regulation tennis court, and it's not the first one.

An exclusive waiting room built for the royal family in the 1920s is hidden in Milan’s central station.

The beautiful tunnel that ran from the lobby to Penn Station is still hidden underneath 34th Street.

An abandoned jewel of a ballroom on the top floor of Australia’s busiest rail station echoes with the sounds of a bygone era.

Natural time capsule containing an alien ecosystem.

A scrap-metal sculpture recalls an infamous 19th-century camel.

This cliff-face port is considered a masterpiece of mining engineering.

Part of a highway network meant to connect Miami to Montreal, this 10-mile stretch of road made from red bricks is still around 100 years later.

Monty Python fans will recognize this abandoned mine as the Cave of the Killer Rabbit of Caerbannog.

The main gates have been locked since 1745 and will remain so until the Stuart Dynasty returns to the throne.

Ride a boat through this magnificent underground world, which now doubles as a theater.

You can row a boat through this ancient underground reservoir.

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

The randomness of this wall of lava lamps helps encrypt up to 10 percent of the internet.

This unusual building straddles two countries.

The hidden offices underneath Memorial Bridge have been locked up since 1976.

The final resting place for the famed creator and narrator of "The Twilight Zone."

A concrete mushroom that divided Stockholm residents for nearly a century.

A gargantuan crystal kingdom.

This abandoned island off the Venetian coast has been a radical monastery, a political prison, and a secret Nazi base.