This mysterious structure by the Washington Monument is a flood barrier designed to protect the White House against rising waters.

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

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

Four bald cypress trees in Lafayette Square, across from the White House, mark the southern edge of a 100,000 year old cypress swamp.

The first purpose-built art gallery in the United States is once again open as a center of craft arts.

The hippo that stands on this university campus was once a drunken flea market purchase.

Garments from across the globe call this hidden gem home.

A miniature rainforest is growing inside of a Midtown Manhattan office building.

What happens here stays here – unless it affects the whole world.

Mysterious installations bring levity and light to a sometimes stodgy city.

A surprising, but overlooked example of geology in the middle of the nation’s capital.

There's a crumbling old mansion inside the Smithsonian National Zoo.

A celebrity from the late Cretaceous period.

The iconic D.C. restaurant where the Cuban Missile Crisis was negotiated, now a Walgreens.

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

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

Largest Roman Catholic church in North America.

An overlooked oasis of quiet on the grounds of Washington's monumental basilica.

This house was once the location of Frelinghuysen University.

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

A portal into the bricked up canal that runs through the heart of Washington D.C.

Before Nixon, "watergate" meant canals.

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

This bridge over D.C.'s Rock Creek Park is sometimes called the "Buffalo Bridge" because of its four buffalo sculptures, which were cast from a single piece of bronze.