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

A derelict bit of transportation infrastructure hidden in the woods.

A multiversal transit station located right below a highway interchange.

Isak Heartstone, a 15-foot wooden troll made out of recycled materials.

An abandoned church gets a second life as a massive beer hall and event space.

One of the U.S. Forest Service's only mansions is outfitted with a dinner table for floating food.

A tiny theater hidden in an alley plays a constant stream of short silent films for anyone willing to peer through the peephole.

Edison's cast-in-place concrete houses were a failure, but way ahead of their time.

The discovery site of the "Capitalsaurus," the official dinosaur of Washington, D.C.

A Spiritualist house of worship where believers communicate with the dead in the spirit world.

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

More than 500 unique and specially designed cases collected by D.C.'s "Harmonica Case Man."

During Prohibition, the U.S. Congress had an "official" bootlegger, with his own Capitol Hill office.

This robust fence in front of a historic Georgetown home is likely made from hundreds of recycled Revolutionary War firearms.

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

The school tested mustard gas for the U.S. Army during World War I.

There's a crumbling old mansion inside the Smithsonian National Zoo.

The oldest continuously operating fish market in the United States.

Before Nixon, "watergate" meant canals.

Once the supplier for noteworthy projects like the National Cathedral, this old brickworks now lies abandoned.

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

A historic ruin just 20 feet away from a busy highway onramp.

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

A wondrous old library overlooking the nation's capital.