This abandoned subway station beneath the Waldorf-Astoria hotel is allegedly used to secretly transport presidents.

The hidden "ballroom" under a mansion lake was built by a criminal who ate a cyanide pill during his trial.

An ancient stone fortress considered by many to be the eighth wonder of the world.

A cavernous three-story, 43,800-square-foot basement that was forgotten about for 60 years.

Under a glass sidewalk lie the remains of some of Manhattan's oldest buildings.

This encrypted monolith protected a secret of Norse mythology by hiding in a church wall for over a millennium.

The abandoned mine where American troops discovered a treasure trove of art and artifacts hidden by the Nazis.

Despite centuries of architectural innovation, this French castle is being built like it's the 13th century.

The once forbidden "secret cabinet" of erotic art from excavations of Pompeii and Herculaneum.

This sword isn't waiting for a king to pull it out. It was placed there by a saint.

There's a 12-foot-tall replica of the Washington Monument hidden under a manhole nearby.

The medieval castle built as headquarters for the Pope's secretive Knights Templar.

Hidden behind Lincoln's head is a 70-foot-long chamber containing enamel plates documenting American history, sealed in a teakwood box in a titanium vault.

One of the world's tallest waterfalls wasn't revealed to the world at large until 2006.

Three bridges and one giant canyon make up this fantastic Spanish settlement.

The oldest pub in Ireland may also be the oldest surviving bar in the entire world.

Over 4,700 miles of trees being planted to hold back the desert.