The world's largest man-made underground caverns offer unparalleled underwater wonders.

"Congress may voluntarily remain in session throughout the summer, in order that our Congressmen may be protected from the intolerable discomforts and dangers of the ordinary outdoor weather!”

The only Leonardo Da Vinci painting in the Western Hemisphere.

Railroad officials in the early 1900s sought to spare travelers the sight of Roman soldiers’ private parts.

The 5.5-mile ring of steel posts around the Capitol Building is one of the largest (and most uniform) of its kind in the world.

A 1,100-ton train fell through the floor in 1953. Workers got it patched up in just 72 hours.

A derelict bit of infrastructure from the canal that once ran through D.C. is landlocked in the heart of the city.

The 28,000-year-old specimen is remarkably intact.

A muscular Art Deco monument represents the struggle between regulators and unbridled markets.

Look closely and you’ll notice that the color changes a third of the way up the tower.

The preamble to the U.S. Constitution written entirely from vanity license plates hangs in the Smithsonian museum.

A little marble compass above George Washington's (empty) tomb in the Capitol marks where D.C.'s four quadrants intersect.

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

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

Those with a sharp eye can find the hidden image of a German Shepherd on the memorial's Mural Wall.

Rumor has it the uncarved lump behind the three famous suffragists is reserved for the first woman president.

Perhaps the most scandalous statue of America's first president.

A hidden gem on the lawn of the U.S. Capitol Building designed by landscape architect Frederick Law Olmsted.

An extravagant western hotel where every floor is the "ground floor."

17th century streets hidden under Edinburgh were once a breeding ground for the black death.

An unusual round church in London with a Templar past.

The signs of a long buried road can still be found hidden beneath a London sewer grate.

An alleyway plaque is all that remains of the independent Texas' envoy that forgot to pay their rent for over a century.

A statue in the heart of Edinburgh honors a bear that served in the Polish military during World War II.