The legendary building that needed ghostly approval before being built.

Crypt mausoleum in cathedral's basement contains relics of third century Roman martyr, Saint Vibiana.

A landmark for a trailblazer for female architects and the designer of one of the most famous and opulent buildings in California.

An original apothecary from 1792.

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

The narrowest house in America is seven feet of pure spite.

Once forgotten and built over, this historic Black cemetery now houses a poignant memorial.

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

The National Park Service’s best-kept secret with views that rival any in D.C.

Daredevil repair workers can worm their way out the access hatch, loop ropes over the apex and rappel down the monument.

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

A tiny monument to the unsuccessful attempt by Thomas Jefferson to place the prime meridian in Washington.

There’s a hidden military meme engraved on the World War II Memorial.

The beloved statue at the National Academy of Sciences is oh so inviting to sit on.

The national park was once a plantation estate.

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

Rumor has it the uncarved lump behind the three famous suffragists is reserved for the first woman president.

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

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

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

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

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

A tribute to the pop culture icon overlooks this Washington D.C. intersection.