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

These recently uncovered walls are all that's left of Washington, D.C's first defense contractor.

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

Largest Roman Catholic church in North America.

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

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

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.

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

A derelict bit of transportation infrastructure hidden in the woods.

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

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

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

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

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

A wondrous old library overlooking the nation's capital.

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

The only museum of sake in the United States is nestled in an unlikely California college town.

An amazing collection of fantastic reptiles, amphibians, spiders, and insects.

An abandoned convention space that has been left to the homeless and the radical.

The Flatiron Building's storied West Coast cousin is a lot more into religion.

An abandoned car and strange wooden debris are among the scattered remains of the old sand and gravel quarry.

A massive ultra-modern church presents a science-fiction view of Catholicism's inclusive future.

This beautiful Victorian ironwork masterpiece was built to keep London from smelling like poop.

An out-of-place grove of sequoias thrives in northern Spain.