The "Key of Keys" for Washington, D.C. and the apocryphal landing place for the military campaign that started George Washington's career is now at the bottom of a well.

The first memorial dedicated solely to the Black troops who fought for the Union.

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

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

These recently uncovered walls are all that's left of Washington, D.C's first defense contractor.

Sit where JFK proposed to Jackie or where Nixon dined on meatloaf at this storied pub.

A celebrity from the late Cretaceous period.

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

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

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

A lovely aquatic park built by a one-armed Civil War veteran who made a fortune from lotuses.

This mausoleum is the final resting place of one of St. Louis's richest and most tragic families.

400 million-year old limestone caverns nestled in the Ozarks served as shelter and a hideout for cultures throughout North American history.

The "World's Largest Underground Business Complex," a 55,000,000-square-foot city underneath Missouri.

Once an ambitious nature spa, these forest ruins now provide a serene monument to failure.

An uninspiring grass verge transformed into a therapeutic community garden.

A restored World War II Churchill Mark VII Tank now stands guard in Carrickfergus.

This former ice house dates to the early 19th-century and featured an underground chamber.

The stone that rocks no more.

Local legend says there's treasure hidden within its crowded graveyard.

An Irish cemetery filled with victims of political strife and violent conflict, including the grave of Bobby Sands.

A brisk climb up this solitary peak leads to the origin story of a towering figure in Irish history.

The inscribed stone at Kilnasaggart is a remnant from nearly a millennium ago.

The lake that fills and empties itself at random.