There are two problems with using sewers for networking
Firstly, old Victorian sewers are often badly maintained, leading to unexpected collapses that are liable to sever the fibre. (In one case I know of, an entire double decker bus vanished below road level.)
Secondly, rats. They just love chewing on anything and everything. When Cambridge University installed their private fibre+copper network some years ago one of the key dimensions in the design was the widest a large rat could open its jaws. All fibre & cables were put into pipes larger than that.
Biting the hand that feeds IT
