The enormous 250-room Vanderbilt mansion conceals hidden doors and secret passageways.

Enormous lake at the bottom of a unique cave system.

Legend has it the commander of the defenders drew a fateful line in the dirt with his sword just before the final battle.

Two small markers remember the Alamo's feline protectors.

Steep canyon walls hide a forested oasis in the West Texas desert.

Equal parts record-breaking temple to consumerism and bizarre fun park, it's more than you think it is...

The world's largest mountain carving could fit all of Mount Rushmore inside it many times over.

This statue of the "father of television" stands prominently in the United States Capitol.

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.

Corn-inthian columns with a uniquely American take on neoclassical architecture.

An ear-piercing buzzer calls lawmakers to a vote with a series of long and short rings to form coded messages.

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

A museum cafe showcases Native American dishes and indigenous ingredients from across the Western Hemisphere.

A heroic pigeon that, through a barrage of gunfire, delivered a message that saved over 100 lives in World War I.

Washington's iconic carousel has a nice piece of Civil Rights history.

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

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

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

The "lovely red Vega" of the legendary record-settling pilot.

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

The actual model from the original "Star Trek" series is now on display at the National Air and Space Museum.

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

Home to items never before seen by the public.

Quite simply the largest tree in the world (by volume).