Large cities have underground networks of pipes and/or wires for water, sewage, natural gas, electricity and telecommunication services. These networks carry heavy fixed costs that must be shared among users. This problem has specific characteristics that we illustrate with an example. After reviewing proportional cost sharing methods, including the one based on the length of the pipes, we present two cost sharing methods that appear to be more interesting in such contexts, namely the Shapley-Shubik and the serial cost sharing rules. We review the properties satisfied by these methods and we discuss the data required for their application. We conclude by recommending the serial cost sharing rule, which seems easier to apply and more appropriate in the context of an underground network of pipes of all sorts