One of the hidden glories of Victorian engineering is proper drains. Isolating a city's effluent and shipping it away in underground sewers has probably saved more lives than any medical procedure except vaccination. But out of sight is out of mind. And that, together with the inherent yuckiness of the subject, means that many old sewers have been neglected and are in dire need of repair. If that repair does not come in time, the result is noxious and potentially hazardous. All this neglect, though, makes it hard to know where best to apply the sticking-plasters. So Mark Hernandez of the University of Colorado, Boulder, and his team have been looking for an easily measurable signal that something is about to go wrong. A candidate, Dr. Hernandez suspected, is hydrogen sulphide. This is one of the gases that make sewage stink. Though not itself damaging to a pipe's fabric, it can be converted by certain species of bacteria into sulphuric acid. Pretty well all sewage smells of hydrogen sulphide, though, so for it to be a useful telltale you need to know just how much is a sign of trouble. Dr. Hernandez and his colleagues therefore collected samples from 36 sewers in various states of decay and started looking. One of their interests was in the mixture of bacteria found in pipes in different states of repair. Rather than culturing these, a process to which not all species are amenable, they ran them through a mass DNA screening that shows up everything in a sample. They also measured the acidity of the sewage soaked into the pipe wall near where the sample was collected and recorded the concentrations in the air there of hydrogen sulphide, methane and carbon dioxide.