Giant acorn-shaped 19th century gazebo from which suburban Washingtonians gazed upon the original "silver" spring.

The only Civil War battle in Washington D.C. took place near this highest natural point in the city.

An obscure federal monument honors the French diplomat who served as Ambassador to the U.S. during WWI.

A Civil War fort in the middle of Washington, D.C. has been swallowed by a forest.

Enormous piles of historically significant stones, dumped by Congress in a forest, and abandoned for 60 years.

The United States Capitol's former columns still stand.

An original apothecary from 1792.

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

This grave marks the passing of a historic Jane Doe whose identity remains a mystery to this day.

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

America's first federal monument.

A museum designed by and dedicated to P.T. Barnum in the town where he is buried.

This nearly endless university hallway is home to a unique sunset known as "MIThenge."

Quirky castle designed by the American actor best known for portraying Sherlock Holmes on stage.

The US Navy's only submarine museum is also home to the world's first nuclear submarine.

Holding multiple historical boats and an entire 19th century village, this New England attraction is the largest maritime museum in the world.

This small-town pizzeria won the Hollywood lottery.

Three and a half miles of cliffs, rocky beaches, Gilded Age mansions and 40 Steps to nowhere in particular.

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

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

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.

A hidden gem on the lawn of the U.S. Capitol Building designed by landscape architect Frederick Law Olmsted.

An unassuming, powerful monument north of the U.S. Capitol bears witness to the resilience of Japanese Americans during a time of grave injustice.

A much-maligned monument to teetotalism.