A whimsical village of unique treehouses resides in a Washington forest.

Visit the site of a subway powered by nothing more than air--and lots of it.

Deep under Manhattan is the largest construction project in city history. So far, the project has taken the lives of 23 workers, or "a man a mile."

An architectural oddity of six stories, accessed by graceful ramps.

A Beaux-Arts triumph set to become Ford's innovation campus

An ornate Detroit church, left empty and ravaged by time.

Abandoned concert venue legendary for its rock counterculture.

A stunning, abandoned limestone quarry in southern Indiana provided the stone to erect the Empire State Building.

Small, privately owned island divided between Michigan and Ohio with abandoned vacation homes and a derelict lighthouse.

Urban explorers embraced America's now demolished first suburban mall, south of Chicago.

An abandoned Gold Coast-era mansion once intended for the exiled ruler of Albania, who never arrived.

This fairy tale-themed amusement park is now abandoned, and childhood metaphors abound.

Underground tunnels remain beneath this abandoned mental asylum.

Beneath the mossy overgrowth covering this once-sprawling campus lies decades of history and the secrets that come with it.

A formerly abandoned mental institute, now back in business.

The oldest surviving bridge in New York City, which reopened to pedestrians in 2015.

A tunnel once abandoned now thrives with graffiti art as an off-limits makeshift gallery.

The feral felines of Roosevelt Island have a small army of allies and an abandoned hospital all to themselves.

Site of the first pathological and bacteriological research facility in the United States.

A crumbling hospital from the 1850s on Roosevelt Island.

Abandoned power station once home to a squatter community, now being developed into an arts center.

Two monstrous warehouses. One now holds priceless works of art, the other stands abandoned and dilapidated.

A modern zoetrope on the New York City subway.

Abandoned 19th century homes once occupied by naval officers, soon to be demolished.