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

The bridge's highly-contested plaque (once again) honors Confederate President Jefferson Davis.

Once home to seven different roller coasters, Glen Echo has undergone many transformations since its founding in 1891.

The semi-reclaimed ruins of this canal were once part of an ambitious plan to reroute the mighty potomac.

This advertisement for talcum powder was painted on the rock face and has since defied removal.

Time travel with sweets across history at this research-based candy store.

A secretive collection featuring objects that were designed specifically for U.S. intelligence operations.

A beach made of semi-precious green sand on the Big Island of Hawai'i.

Race through the maze, get your name on the sign.

A priest with no formal artistic training used basic house paint to create gorgeous religious scenes inside this historic church.

A unique set of caves that spits waves back out into the ocean.

This Art Deco fortress in upstate New York gives Manhattan's vaunted examples of the style a run for their money.

The innocent intentions of this monument are somewhat lost in the sculpture's muscled imagery.

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

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

The 5.5-mile ring of steel posts around the Capitol Building is one of the largest (and most uniform) of its kind in the world.

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

Washington's iconic carousel has a nice piece of Civil Rights history.

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

Home to items never before seen by the public.

The land is home to swimming ponies and a legendary 18th-century treasure.

The taxidermy remains of the most popular pony in children's literature.

Down the road from where the Wright brothers lifted off is another flight tribute practically hidden from sight in the woods.

The Cadillac Ranch for monster truck aficionados.