Originally conceived as part of a proposed Arizona State Capitol building in the 1950s, this structure built in 2004 stands as a monument to the architect's legacy.

A spiritual vortex among the red rocks in Sedona.

This famous bridge twisted itself into oblivion, but not for the reason you might think.

The highest point in Washington state is a volcano covered in glaciers.

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

A former major munitions plant that has been converted into a sprawling art space.

An organ located deep within a cave, whose "pipes" are the geological features of the cave itself.

This rare triple-jurisdiction drawbridge passes through Virginia, Maryland, and Washington, D.C.

This fort down the Potomac from Washington, D.C. was once the only defensive fort protecting the capital.

Once home to seven different roller coasters, Glen Echo has undergone many transformations since its founding in 1891.

This cannon dating to 1755 has stood guard at this suburban intersection since 1915.

Built in the 1850s, this church was of the first five churches constructed in Harper's Ferry.

A dozen U.S. presidents have stayed at the historic resort, which was known as James Buchanan’s “Summer White House” from 1857-1861.

The world's only teal arches exist in a city that cares more about its aesthetic than an Instagram influencer.

Enormous lake at the bottom of a unique cave system.

This isn't your average roadway—it's actually a National Park and a transportation pioneer.

America's Most Wanted's set resides in this tribute to the history of crime and punishment.

An amazing example of early-20th-century architecture, this church features marble transported from Utah.

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.

The monument's pointy aluminum tip has been melted down by repeated lightning strikes.

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

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

A traveling postal dog covered 48 states and more than 140,000 miles, and he lives on as taxidermy, patched up with a rabbit's foot and a pig's ear.

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