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

The remains of a failed attempt at medieval nerdery are hidden deep in the woods of Virginia.

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

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

The first full-scale tunnel built in America is short but scenic.

A mysterious lake provides fresh water without any source, somehow restoring itself from atop its mountaintop perch.

An eerie, derelict amusement park in Berlin.

100 years later, they're still down there.

One of the last surviving C-97s sits in a field in front of a Wisconsin inn.

A bizarre house filled with an astounding array of collections.

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

Members of Congress have traveled between the buildings on Capitol Hill for a century hidden from tourists, press, and storm clouds.

Despite its lavish ornamentation, and wholesome lifestyle of its devotees, this mountain commune has faced its share of issues.

This abandoned dream castle and its cemetery look out over the city that once inspired their creation.

The famous boat that once ferried Bogart and Hepburn still floats in Key Largo after being abandoned for years.

When this test site was abandoned, they didn't even bother to take the rocket with them.

A mysterious castle built as a monument to lost love.

A cave on the Maryland border where civilians took refuge during the bloodiest battle of the Civil War.

A star fort along the C&O Canal in western Maryland that protected America in three wars.

Despite violent work crews and massive delays this abandoned canal tunnel was built to last into the modern day.

An abandoned stretch of highway, deteriorating next to the operational one.

A 70-foot-deep ice cave formed by lava from Mt. Fuji on the edge of Japan's "Suicide Forest." 

A little-known subterranean street runs under some of the best-known buildings in Chicago's River North.

A century-old network of freight tunnels underneath the city's historic business district.