Largest Roman Catholic church in North America.

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

The national park was once a plantation estate.

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

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.

The Byzantine, pre-Columbian, and medieval art at this stately mansion are some of the most under-appreciated collections in D.C.

During the Industrial Revolution this “Temple of Invention” was full of intricate miniature machines and gadgets.

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

Franciscan monks created a facsimile of the Holy Land for North Americans who couldn’t afford the trip overseas.

The United States Capitol's former columns still stand.

Home to items never before seen by the public.

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.

A historic neighborhood church is reborn as a psychedelic arts collective.

A painfully obvious spy house sits right across the street from the Russian Embassy.

Headquarters of the legendary clairvoyant's readings, spiritual hospital, and massage school.

A lightship turned museum, outfitted with artifacts from the vessel's past as part of the U.S. Lighthouse Service.

Get up close and personal at this Fresnel Lens.

Rowdy rebels fortified this home during the first popular uprising in United States history.

Captain John Smith thought these cliffs were amazing in 1608 but sharks thought so 20 million years before him.

An isolated island home to one of the oldest English-speaking communities in the region and Maryland's state dessert.

After hundreds of years of settlement, this once-bustling fishing village sunk into the Chesapeake Bay.

This scenic Maryland park was the site of one of the worst prison camps of the Civil War.

Inspired by a foil-wrapped potato, (seriously) the Markel Building has been deemed one of the 10 ugliest buildings in the world.