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

This stately building in Alexandria, Virginia was built by Freemasons to honor one of their most famous members.

Long buried under the 20th-century landscape, this lift lock of the Alexandria Canal is the lone remnant of an ambitious early American transportation project.

This twice-relocated "Usonian" home is among the smallest built by master architect Frank Lloyd Wright.

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

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

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

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

D.C.'s first central library was born out of a chance encounter with the philanthropist whose name it bears.

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

The innocent intentions of this monument are somewhat lost in the sculpture's muscled imagery.

A monument in Washington D.C. marks the spot from which all other roads were supposed to stem.

The grand marble structure next to the White House is Andrew Carnegie's temple to Pan-American diplomacy.

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

The oldest continuously operating fish market in the United States.

At 207 feet, one of the world's longest continuous escalators.

This seemingly out-of-place gas station by the Watergate hotel was once described as the most expensive gas station in the world.

The little-known, 300-year history of the area includes former lives as a bustling tobacco port, parking lot, and industrial dump.

The house that Frank Lloyd Wright "(shook) out of his sleeve at will" at the age of 86.

Frank Lloyd Wright's most iconic home dangles over a Pennsylvania waterfall.

The legacy of a wealthy executive and his wife, this little-known art museum features works by Picasso, Monet, Renoir, Cezanne, and more.

The end of the 4 Line is also the end of the line for 300,000 souls in one of NYC's most illustrious cemeteries.

This hidden oasis features a stunning view of the Hudson River and the Palisades from a castle-like museum.

Visit the monolithic tomb of America's most famous general.