North Brother Island, New York.
This abandoned 20-acre island sits amazingly close to the bustling center of New York City yet is completely unused. It was home to a hospital in the 19th Century, then housed veterans after World War II before becoming one of the first drug treatment centers for teens in the 1950s. Corruption and failure caused the facility to close and the island has since been off limits to the public, though some urban explorers have made their way onto it anyway. The island was also the site of an infamous shipwreck in 1904 in which over 1,000 people drowned or burned to death.