The Pentagon sits on a former island that was in Virginia at low tide and D.C. at high tide.

The hidden offices underneath Memorial Bridge have been locked up since 1976.

Go take a picture with the inventor of the daguerreotype photographic process.

This dig site outside D.C. is known for its exceptionally high density of baby dinosaur fossils and dinosaur eggs.

First a horse rendering plant, then a 19th century landfill, this beach is full of glass from thousands upon thousands of broken bottles.

A railroad tunnel that, upon its completion, was considered one of the engineering marvels of the modern world.

America's second-oldest brick baseball stadium was home to a legendary WWII series that only sailors got to see.

Nine of the very few concrete ships ever made in the U.S. are beautifully decaying off a Virginia pier.

A museum dedicated exclusively to aircraft used in the military.

The remnants of a wooden-hulled fleet built and abandoned in WWI, now home to a thriving ecosystem.

This ambitious attraction was the brain child of a lake resort impresario, and built by the WPA.

Bring your own cardboard to the slippery slopes of concrete hidden in a neighborhood park.

A pair of slides and a tree-lined stair corridor have been an urban oasis for nearly 40 years.

More than a century of radio history is told in these displays.

A strange egg in a mucky bog of western Ireland marks the spot of an aviation milestone.

America's first federal monument.

The only diamond mine in the world where you can keep what you find.

A once-booming Mississippi River town with a history of racial violence is now eerie and mostly abandoned.

The site of a 19th century "dream city" that almost rivaled Chicago before being swallowed by the Indiana sand dunes.

One of America's first spies started out as a teacher in this little red one-room schoolhouse.

One of the most influential presidents in U.S. history wanted only this plain, elegant monument as his lasting memorial.

American involvement in World War I officially ended in 1921 in New Jersey, three years late and thousands of miles from the battlefield.

This sandwich shop has a century-old elevator behind a sheet of plexiglass.

The cave where two British judges hid in exile after sentencing the king to death.