An organ located deep within a cave, whose "pipes" are the geological features of the cave itself.

A clone-tree in Washington Square Park sprung from seeds that went to the Moon and back.

Berthed at the Charlestown Navy Yard in Boston, "Old Ironsides" is the oldest commissioned warship still afloat.

A tribute to Ponce de Leon's supposed quest for eternal youth.

I-95's Tijuana-style celebration of kitsch at the Carolina border.

A lawn full of retired spacecraft on display at one of NASA's first research labs.

These unusual rooms on wheels are holdovers from the 1960s.

This stately building in Alexandria, Virginia was built by Freemasons to honor one of their most famous members.

Thomas Jefferson's estate is home to hundreds of varieties of historic fruits and vegetables.

Forget Hollywood, one of the biggest stars in the world can be found in Roanoke, Virginia.

This Virginia railway overpass is said to be haunted by a serial killer in a rabbit costume.

The romance of early commercial flight still fills this Art Deco destination.

This isn't your average roadway—it's actually a National Park and a transportation pioneer.

Coins and medals straight from the mint await visitors to this shop in the nation's capital.

A derelict bit of infrastructure from the canal that once ran through D.C. is landlocked in the heart of the city.

"Congress may voluntarily remain in session throughout the summer, in order that our Congressmen may be protected from the intolerable discomforts and dangers of the ordinary outdoor weather!”

A little marble compass above George Washington's (empty) tomb in the Capitol marks where D.C.'s four quadrants intersect.

Those with a sharp eye can find the hidden image of a German Shepherd on the memorial's Mural Wall.

Rumor has it the uncarved lump behind the three famous suffragists is reserved for the first woman president.

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

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.

One of the oldest train stations still standing in Virginia played a vital role during the Civil War.

The most historic church in Fairfax County played a role in the creation of the American Red Cross.