An abandoned stretch of highway, deteriorating next to the operational one.

A museum commemorating the Donora Smog, which killed 20 people in October 1948 and sparked national outrage over air pollution.

This lush forest valley was once mined for lumber to the extent that it was once nicknamed "The Pennsylvania Desert."

A "house to which to flee" for settlers in colonial Pennsylvania.

Buildings that dynamite couldn't bring down were abandoned because of bathroom issues.

These disused steel furnaces are now haunting ruins that may one day be a national park.

North America's oldest quarantine facility was built in response to Philadelphia's 1793 yellow fever outbreak.

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

Dr Barnes' $25 billion private collection of art and arboretum.