The Essex County Jail was first built in 1837 by John Havilland, who had also built some other now abandoned jails, all which focused on prison reform. The prison was constructed when a courthouse was destroyed by a fire. The building was two stories, and in 1890, 1895, 1904, 1909, they continued to build more onto the jail. New buildings were added like hospital, powerhouse, and laundry building. The cell blocks were for temporarily holding inmates waiting for trials or inmates that were held for petty crimes. There were about 300 cell blocks, though over crowding led to 500 inmates. Also, in 1991 scenes for a movie "Malcom X" were shot at the jail. The prison was closed 1970, with a complete of a new jail. When the jail became abandoned, homeless drug addicts moved in, and still may live there today, Two bodies of drug users have also been found on site.
I would not recommend visiting or going inside, there still may be people living here, and if not there is probably a large police presence to make sure the homeless don't make it their home again.