The Cold War-era "underground Pentagon" is a sprawling subterranean hideaway for the Defense Department.

This extraordinary Depression-era expenditure was modeled after a postcard of a Mediterranean hotel.

This national landmark is also home to a massive treehouse.

An abandoned incline railway site is now a green space with six acres of trails connecting hilltop neighborhoods.

The lasting remains of the region's once booming cement industry.

One of the few remaining covered bridges in Lancaster County.

Considered by many to be a local landmark and treasure.

A piece of crumbling highway infrastructure was repurposed as a high-tech facility for testing racecars.

Hidden in the hills of Western Pennsylvania lies one of the best-kept ghost towns in America that you can legally visit.

These unique rock formations were created during the Ice Age.

Designed by Horace Trumbauer in 1893, this Gothic-style castle is now a home for academics.

An abandoned railroad tunnel transformed into a creepy path for hikers and cyclists.

Each step is a mere four inches tall, said to be measured by the stride of a woman wearing a hobble skirt.

This massive sycamore tree has been labeled as one of the largest in the state.

Narrow gorges and deep pools created by long-ago glaciers.

"Where the ducks walk on fish."

For decades, it has disappeared and reappeared as water levels rise and fall.

What looks like a simple quaint estate in rural Pennsylvania was in the 18th century home to a small monastic community.

One of the oldest log cabins in America is one of the last of its kind this historic Swedish dwelling remains a piece of living history.

Pennsylvania's highest point is spotted with boulders which were pushed to the surface by nothing but cold.

A rock maze opens up to a panoramic viewpoint at an abandoned mine full of ghost stories.

A tradition unlike any other, this marks the site of the annual Groundhog's Day tradition.

A field of nothing but boulders which resembles a giant's gravel driveway.

This stone tower marks a lookout point over the historic Washington Crossing, and maybe some buried treasure.