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

This sprawling farm was once a Cold War-era hideaway for the nation's top diplomats.

Collapsing castle on the Hudson river was once the storage site of military surplus business.

Accessible only by boat, this centuries-old island fort never saw troops or combat.

A terrific purveyor of natural history objects and curios.

This ruined quarry once produced the distinctive red sandstone used for construction projects in Washington, D.C.

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

Fragments of architectural wonders from around the globe are embedded in the newspaper building's walls.

The palatial McDonald's that inspired Wesley Willis.

Each of these gorgeous, intricately decorated rooms is painstakingly crafted to the scale of 1 inch: 1 foot.

Ironically this massive termite is the mascot for a company that is acclaimed for killing his kind.

Three and a half miles of cliffs, rocky beaches, Gilded Age mansions and 40 Steps to nowhere in particular.

Books bound in human skin and H. P. Lovecraft's letters are among a few of the treasures this library has to offer.

A haunting exhibit in a wildlife refuge center highlights threatened species and the conservation efforts needed to save them.

Built by the patriotic residents of Boonsboro in a single day.

This fort down the Potomac from Washington, D.C. was once the only defensive fort protecting the capital.

The land is home to swimming ponies and a legendary 18th-century treasure.

An astounding collection of medical specimens, including bits of President Lincoln.

At Washington's Dulles Airport is a satellite museum (no pun intended) with three quarters of a million square feet of aircraft history.

The romance of early commercial flight still fills this Art Deco destination.

Herbert Hoover's "Brown House" rural presidential retreat was a PR disaster amid the start of the Great Depression.

This isn't your average roadway—it's actually a National Park and a transportation pioneer.

This unique historical monument features large concrete spheres designed to resemble fallen stars.

In upstate New York, gorgeous waterfalls and a watermill are slowly being reclaimed by nature.