There is an amazing post about sewer systems under the big cities of Europe, America and Australian over at BDLGBLOG (amazing pictures as well!). It argues that the complexities of these sewer/tunnel systems is best being understood as more or less complex mathematical systems (knot theory):
Manchester's storm overflow sewers, the rumor goes, are actually topological models. They are knot theory in built form.
Other rumors claim that a former student of that program is now Chief Engineer for the city of Brisbane, Australia, where he leads the construction of new civic infrastructure; every sewer and spillway built there is designed by him alone. As a result, each time you flush a toilet in Brisbane, a bewildering and exhaustively contorted world of concrete knots and brick culverts comes to life, engineered to faultless precision, washing everyone's waste out to sea.
Manifolds, loops, toroids, even prime number sequences: the entire history of Western mathematics can be derived from the sewers of Brisbane, monuments of urban plumbing.
Whole thing reminds me of one of my favorite ever movies called Moebius in which a train on the subway system of Buenos Aires suddenly disappears and a mathematician is called in to examine the mystery. although the movie is student project of the Universidad de Cine in Buenos Aires it is extremely beautiful (if you like subway tunnels that is…)
It used to be pretty much unavailable (i even went to the Universidad de Cine and they did not have copies for sale) but it seems to be available as a torrent.