A computer bug so primitive it was an actual insect.

Cold War intelligence agents monitored communist embassies from an attic in a former pigeon coop.

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

An atomic bomb-proof strongbox protects the U.S. Constitution from terrorists and thieves.

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

The famous abolitionist’s preserved estate is one of Washington's finest monuments to its great Black citizens.

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

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

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

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

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

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

A nostalgic bibliographic gem.

People escaping slavery may have hidden inside a corpse vault.

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.

A historic estate packed with George Washington's heirlooms, and its own nuclear bunker.

The winding route passes through an enclosed alleyway, two tunnels, and leads to the White House basement.

A derelict bit of transportation infrastructure hidden in the woods.

During the Industrial Revolution this “Temple of Invention” was full of intricate miniature machines and gadgets.

A cavernous three-story, 43,800-square-foot basement that was forgotten about for 60 years.

A painfully obvious spy house sits right across the street from the Russian Embassy.

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

A wondrous old library overlooking the nation's capital.

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