Went to a closed up mental hospital in my city with some friends at night. Everything was cool until we got to the basement, where there were 5 meter wide and 4 meter high lit tunnels going far to both directions. We headed down one end and found a huge active generator, then we went the other way and as we were about to hit another intersection, we saw two men wearing full green attire and face masks walking at the end of one of the corridors at 2 AM. We got out of there real quick.