Glenn Dale Hospital
photo from The Glenn Dale Hospital Mission
Glenn Dale opened in the same era as Fairfield State – the 1930s was a popular time for mental institutes, apparently. Well, actually, Glenn Dale wasn’t originally used for that purpose – it was a tuberculosis hospital with one building for adults and one for children. Eventually the tuberculosis problem died down and Glenn Dale was repurposed. It closed in 1982 due to asbestos and structural problems, but before it closed it was (supposedly) home to the criminally insane. As with Fairfield State, the buildings are connected via underground passageways, which people have been exploring since the day Glenn Dale officially closed its doors.
Exploration might not be the best idea, though, and not just because of the asbestos (although that should be an obvious deterrent). One rumor says that when the hospital closed, the remaining patients were just turned loose. Having nowhere else to go, many of them simply broke back into the abandoned buildings and lurk there even today.
Another story goes that a police officer went to check out the buildings himself after getting a call that the buildings were being vandalized by a bunch of kids. After he went in, someone in the vicinity heard gun shots and called the police. When the police arrived, they found the first officer standing in one of the rooms, staring straight ahead at nothing. He had emptied his gun firing at something that no one ever found.
Source: Neatorama's Five hospitals you don't want to check into