An Industrial Revolution-era public work that purified water using nothing but sand.

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

This stunning blue and gold room changed cities twice before becoming part of the Smithsonian.

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

This lonely waterfront memorial to the men of the Titanic was erected by the "Women of America."

People escaping slavery may have hidden inside a corpse vault.

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

A historic estate packed with George Washington's heirlooms, and its own nuclear bunker.

A three-story tower of books about Abraham Lincoln is one of the more unusual monuments to the president.

The winding route passes through an enclosed alleyway, two tunnels, and leads to the White House basement.

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

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

A derelict bit of transportation infrastructure hidden in the woods.

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

A lovely aquatic park built by a one-armed Civil War veteran who made a fortune from lotuses.

A cavernous three-story, 43,800-square-foot basement that was forgotten about for 60 years.

A wondrous old library overlooking the nation's capital.

This statue inspired by Renoir's famous painting embodies the jovial spirit of Key West.

Bat house that has only ever housed bat for one night; they all flew away and never came back.

This former prison is an Art Deco masterpiece with a past full of civil rights flashpoints.

An octet of classic chrome trailers speared into the ground act as a tribute to the silver bullets of the open road.

A mega theme park-wax museum mélange containing all things biblical.

The collection of 80+ pieces created between the years 1100 and 1500 is intended to be seen, heard, smelled, touched, and tasted.

The haunting remains of a failed housing development can still be found behind the big box stores that were to anchor them.