My idea is some what like tranzit, or it could be an extention. So first you start off in a small blank area and there is now way out except having to buy the sewer grate (door) then there is a ladder that takes you down to a little raft now that your down you can go on the sidewalk type of thing on the side of the sewer tunnels. After a while you will eventually find all the pieces to complete the raft. Once bilt enjoy searching the tunnels for perks, guns, parts, etc. P.S. you might be able to swim if wearing scuba gear like in the "Moon". Hope you could take this into consideration and thanks for taking time to read this.