A historic blockhouse and trading post give a glimpse into the early colonial history of South Carolina.

An unfinished rail tunnel that has since been used to house cheese and bats.

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

The walls of the iconic dome are hollow and have a secret stairway.

Fittingly, America's museum of architecture is itself a magnificently designed old building.

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.

One critic described it as "middle Mussolini, early Ramses, and late Neiman-Marcus." Another called it an architectural "natural disaster."

Architecture lovers won’t stop touching the National Gallery's 19.5 degree marble prow.

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

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

This stunning blue and gold room changed cities twice before becoming part of the Smithsonian.

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

The oldest continuously operating fish market in the United States.

The country's oldest continually-operated mini golf course lies hidden in plain sight, eclipsed by one of D.C.'s most popular tourist attractions.

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

One of the dwarven trees dates back to 1625 and survived the Hiroshima bombing.

Once the supplier for noteworthy projects like the National Cathedral, this old brickworks now lies abandoned.

Art deco landmark on the outskirts of Washington, D.C.

The house where John Wilkes Booth conspired with his co-conspirators.

A historic arena where the Beatles played their first concert in the U.S.

An Industrial Revolution-era public work that purified water using nothing but sand.

Largest Roman Catholic church in North America.

A hidden passageway now marks the site of an assassination attempt on Ronald Reagan that some say broke a 140-year-old curse.

A small triangle of DC grass is the final resting place of one of Sonny and Cher's songs.