We found tunnels like this all over our campus (Lehigh University)... we used to explore in the middle of the night.. One time we checked it out during the day and came up on a tunnel in the path to the University Center... my friend was first and popped his head up and someone called out his name.
I guess someone recognized him and was wondering what he was doing in the "sewer". That was a funny scene. A few months later there were locks on most of them, but we still managed to get in.