The hidden offices underneath Memorial Bridge have been locked up since 1976.

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

A traveling postal dog covered 48 states and more than 140,000 miles, and he lives on as taxidermy, patched up with a rabbit's foot and a pig's ear.

The remains of Martha, the last known passenger pigeon, serve as a tool to educate about conservation.

Decades before the scandal, this staircase on the river was a literal "water gate."

The most accurate timepiece in the world.

A celebrity from the late Cretaceous period.

The "lovely red Vega" of the legendary record-settling pilot.

You could be standing at the site of one of D.C.'s most fatal tragedies and not even know it.

The oldest continuously operating fish market in the United States.

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

The national park was once a plantation estate.

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

This stunning blue and gold room changed cities twice before becoming part of the Smithsonian.

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

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

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

The beloved statue at the National Academy of Sciences is oh so inviting to sit on.

The country's oldest continually-operated mini golf course lies hidden in plain sight, eclipsed by one of D.C.'s most popular tourist attractions.

Fittingly, America's museum of architecture is itself a magnificently designed old building.

A derelict bit of transportation infrastructure hidden in the woods.

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

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

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