The actual model from the original "Star Trek" series is now on display at the National Air and Space Museum.

This leafy estate is worth $18 million and is so grand it has two mailing addresses.

An Industrial Revolution-era public work that purified water using nothing but sand.

There’s a hidden military meme engraved on the World War II Memorial.

A nostalgic bibliographic gem.

People escaping slavery may have hidden inside a corpse vault.

The walls of the iconic dome are hollow and have a secret stairway.

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

The winding route passes through an enclosed alleyway, two tunnels, and leads to the White House basement.

A three-story tower of books about Abraham Lincoln is one of the more unusual monuments to the president.

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

Enormous piles of historically significant stones, dumped by Congress in a forest, and abandoned for 60 years.

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

Franciscan monks created a facsimile of the Holy Land for North Americans who couldn’t afford the trip overseas.

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

This long, dark tunnel is open to hikers, but those without lights are known to get confused even on the straight path.

There's something big lurking beneath the surface of this tiny Oregon park.

The trunks of 3,000 year-old trees lurk beneath the surface of this crystal clear lake in Oregon.

Formerly secret, precariously perched boulders.

Mountain known by the Tillamook Tribe as "the place of the supreme deity."

No one knows how this Pacific Northwest spruce tree came to have so many trunks.

This bridge provides safe passage for squirrels.

The only house in Oregon designed by the famous architect, Frank Lloyd Wright.

A plaque commemorates the spot in the Oregon woods where the first-ever Geocache was hidden.