We had one of these tunnel systems in my old neighborhood where I grew up, They were storm drains for rain water, not really sewers for sewage. It was so big, you could ride a bike in, which is what we used to do. It went for miles, so when you got to one of the rooms where the ladders go up, you wouln't really know where you were, and it was great to climb up the ladders and see what part of the neighborhood you were in.

I suggest you get to know these tunnels, and if there is ever a nuclear war, you can start your own civilization of underground dwellers down there. Your choice whether you want to become cannibals or not, of course.