People escaping slavery may have hidden inside a corpse vault.

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

This grave marks the passing of a historic Jane Doe whose identity remains a mystery to this day.

One of just a few cities in the world to base their center of government on an island. Others: Paris and Osaka.

Senators used to relax in the nearly forgotten marble tubs now hidden in the U.S. Capitol Building's basement.

This unacceptably tall building was the real reason for Washington, D.C.'s skyscraper ban.

Members of Congress have traveled between the buildings on Capitol Hill for a century hidden from tourists, press, and storm clouds.

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

Home to items never before seen by the public.

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

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

This picturesque Umbrian city has a subterranean side that has long remained hidden.

The construction of a tram line unveiled hidden remnants of the old medieval city.

The final resting place of up to 6,000 Confederate soldiers, the largest mass grave in the Western Hemisphere.

An ancient burial ground believed to have been used for ritual sacrifice.

This grave in a ghost town cemetery marks the death of a witch on a day that never happened.

The legacy of an unsolved, gruesome murder rests beneath the spans of this historic bridge.

This railroad overpass is said to be home to a murderous goat-man whose very legend has led to very real deaths.

The longest navigable underground river in the United States is home to countless blind and albino animals.

The largest cemetery in Louisville is the final resting place of Col. Sanders and Muhammad Ali.

This abandoned cemetery has a dark past of bodies buried over bodies.

The only building in Louisville (hopefully) that contains two ancient holy skeletons.

Curious cave that stays a cool 56 degrees all year long.

Built in 1869, this now-retired structure looks more like a medieval fortress than a county lock-up.