Powerful nuisance odors from sewage and wastewater treatment facilities are a worldwide problem, but finding and eliminating the sources of such unpleasant aromas can be difficult. Scientists have compared the search to peeling an onion—one layer is found and removed, but then a second and third emerge, ready to make their odoriferous presence known. Scientists have now developed a system to sample, measure, categorize and control these smells and are applying it from Seattle to Sydney.