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

An obscure public service from the U.S. government that redeems burnt, moldy, and soiled old greenbacks.

Government testing at the asylum briefly explored using marijuana as a "truth serum" on Nazi prisoners of war.

The national park was once a plantation estate.

The most decorated dog of World War I is preserved in the Smithsonian.

People escaping slavery may have hidden inside a corpse vault.

A Civil War fort in the middle of Washington, D.C. has been swallowed by a forest.

A nostalgic bibliographic gem.

The Byzantine, pre-Columbian, and medieval art at this stately mansion are some of the most under-appreciated collections in D.C.

A derelict bit of transportation infrastructure hidden in the woods.

A hidden gem on the lawn of the U.S. Capitol Building designed by landscape architect Frederick Law Olmsted.

A lovely aquatic park built by a one-armed Civil War veteran who made a fortune from lotuses.

A monument in Washington D.C. marks the spot from which all other roads were supposed to stem.

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

This grand gothic brewery has been pumping out suds for over a hundred years.

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

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

The site of the climactic scene from the classic horror film is now a historic landmark.

The United States Capitol's former columns still stand.

A diminutive plaque recalls the treetop sniper who almost killed Abe Lincoln.

A rotating cast of guys and dolls in front of a Washington, D.C. building.

When it opened during the final years of the embargo, all the drinks and cigars were free.

A historic neighborhood church is reborn as a psychedelic arts collective.

The oldest continuously operating fish market in the United States.