Metropolitan State Hospital - Waltham, MA
I am not going to go through all the stories because they are extensive and varied, but I will tell you enough so you will go and discover more.
The hospiatal was only recently closed in 1992 after a senator called for an investigation and discovered 19 cases of severe patient abuse and torture. The most notable event was a patient which killed another with a broken glass bottle. The victims body is said to be buried in nine different "packages" throughout the property.
The property is said to be extremely active and dangerous. It is of course patrolled by local law enforcement which for obvious reasons take tresspassing very seriously, but many people still venture in.
The complex itself sprawls over 26 acres with several building all connected with an underground tunnel system. These tunnels were rightfully named "Death Tunnels" as they were the primary route out of the facility. This is where the activity is said to be the most intense.
Many people have been injured literally running away from the property scared half to death.
As I said, there are many stories and since the facility closed in 1992, stories are relayed by those who worked there. A simple google search will yield you many more stories about the facility and why it is so well guarded.