The vast chamber inside this abandoned cooling tower looks like something out of a sci-fi dystopia.

This fairytale castle was built as a show of power but was nearly torn down thanks to its history of torture.

A breathtaking array of ancient Pueblo structures and artifacts counterintuitively preserved by neglect.

The seamy, underground secrets of Portland's criminal past.

A walk through the historic city center, under the streets of Seattle.

The longest continuous lava tube in the continental U.S. offers 2.5 miles of dark explorations for the adventurous.

Series of storm drains connected to a dark urban legend.

This expansive bridge in the Canadian wilderness was originally just built for friends.

A giant Troll lurks under one of Seattle's oldest bridges.

Eons ago, a rushing lava river formed this pristine passage through solid rock.

The ancient dwellings of the Puebloans, located in the cliffs of majestic mesas.

Ninety feet off the valley floor is an ancient dwelling put under protection of the U.S. government by Teddy Roosevelt.

A geological mystery on Camelback Mountain.

The site of the climactic scene from the classic horror film is now a historic landmark.

The remains of Martha, the last known passenger pigeon, serve as a tool to educate about conservation.

Look closely and you’ll notice that the color changes a third of the way up the tower.

Once America's longest-manned military fort, now an abandoned hulk.

A crumbling hospital from the 1850s on Roosevelt Island.

The world’s first penitentiary, meant to be humane, drove men insane.

This ruined fort hidden at the top of Central Park can't seem to keep from losing its historical plaques.

One of only two commuter aerial tramways in the United States.

Hidden in Central Park's Ramble is a troublesome cave buried just beneath the surface.

The 28-foot granite map plots the locations of 40 theaters in New York City.

Lie in a hammock where George Washington stood.