9. Fairfield Hills State Hospital, Newtown, Connecticut

Fairfield Hills State Hospital, Newtown, Connecticut, one of many abandoned hospitals around NYC

The Fairfield Hills State Hospital campus is crossed with a network of underground tunnels that connect the various buildings spread out over 770 acres. Opened in Newtown, Connecticut in 1931 to help alleviate some of the state’s overpopulation issues in its other mental institutions, the campus consists of 16 buildings. The tunnels were mainly used to transport patients between treatments and laboratories, confinement rooms, operating rooms, and in unfortunate cases, the morgue. At its peak, the hospital could hold 4,000 patients. The patient-to-doctor ratio had always been an issue with the facility.

The grounds were partially redeveloped in 2009, long after its closing in 1995. Some of the buildings are being used as offices or for commercial use, but many remain untouched since their last residents were moved to other facilities. Open to the public, exploration of the grounds is highly encouraged, although any vacant buildings are off-limits.