A hoard of sky catalogs, astrophysical journals, even the works of Galileo and Copernicus.

In the basement of the U.S. Capitol Building is a hidden storage room full of ornate floor tiles leftover from the 1850s.

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

Where the U.S. government kept its actual treasure, before Fort Knox.

This sandwich shop has a century-old elevator behind a sheet of plexiglass.

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

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

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.

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

Housing the National Women's Party since 1929, this historic house is now a monument to the fight for gender equality.

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

A computer bug so primitive it was an actual insect.

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

Four bald cypress trees in Lafayette Square, across from the White House, mark the southern edge of a 100,000 year old cypress swamp.

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

People escaping slavery may have hidden inside a corpse vault.

A haunting tribute to pioneering photographer Clover Adams.

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

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

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

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

This imposing Masonic temple a mile from the White House was the first public library in Washington, D.C.

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

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