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

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

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

Captain John Smith thought these cliffs were amazing in 1608 but sharks thought so 20 million years before him.

The last holdout of a pre-Civil War rebel who took the matter of slavery into his own hands.

Rediscovered after a century, the father of the American Navy was reinterred in something dredged up from Davy Jones’ locker.

The privately owned cemetery that holds room for Washington's finest when they step down from life.

America's first federal monument.

A super-sized promotional trick that is now a D.C. landmark.

The house where John Wilkes Booth conspired with his co-conspirators.

Home to items never before seen by the public.

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

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

This pun was simply waiting to come to life, and one defense contractor made it so.

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

The final resting place of the tragic king and queen of the Jazz Age is tucked away in a small Maryland graveyard.

The first purpose-built art gallery in the United States is once again open as a center of craft arts.

At 207 feet, one of the world's longest continuous escalators.

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.

A small but immensely extravagant palace built by the "Night King."

The patron saint of spinsters grasps a container of dried blood.

The only Civil War battle in Washington D.C. took place near this highest natural point in the city.

A tiny piece of the Moon is embedded in this stained glass masterpiece.

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