Concrete strongholds stand guard over the Golden Gate.

Canyon walls shrouded in ferns.

The world's longest known cave system.

The granddaddy of all tourist traps, built on ice water, jackalopes, and a giant dinosaur.

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

A Mexican Romeo-and-Juliet-esque legend has given this super-narrow alleyway a reputation for romance.

Mexico's astounding mummy museum with "the world's smallest mummy."

The tallest The Church of Jesus Christ of Latter-day Saints temple in the world soars above the Beltway.

These unusual rooms on wheels are holdovers from the 1960s.

The tomb was guarded by soldiers from both sides of the war, and some left their initials carved in the brick.

At Washington's Dulles Airport is a satellite museum (no pun intended) with three quarters of a million square feet of aircraft history.

The graveyard holding the remains of George Washington's slaves was forgotten for nearly 200 years.

Soldiers were buried next to Lee's house in the center of Arlington Cemetery to dissuade the general from reclaiming his property after the war.

Collecting 700 years of mechanical death-dealing, the NRA's sprawling gun museum is a revealing testament to the American obsession with armament.

The semi-reclaimed ruins of this canal were once part of an ambitious plan to reroute the mighty potomac.

The Dover International Speedway is home to a towering, concrete-skinned, racecar-hungry behemoth.

World War II-era bunkers are hidden under the beaches of this picturesque park, which is still scattered with wartime watchtowers.

A dramatic, 360-degree recreation of Pickett's Charge in the Civil War.

This lush forest valley was once mined for lumber to the extent that it was once nicknamed "The Pennsylvania Desert."

This palatial botanical garden is home to one of the largest pipe organs ever installed in a private residence, and a piece of Wood's cycad.

Once the "Eighth Wonder of the World," this iron railroad bridge was decimated by a tornado.

This charming colonial alleyway is one of the oldest continuously used residential streets in the U.S.

A toxic ghost town sitting atop a massive coal fire.

Hidden for 165 years inside a building designed by Thomas Jefferson are the remains of what may be America's first educational chemistry lab.