The City of Lake Oswego, Oregon faced difficult questions with their main interceptor sewer running through Oswego Lake being seismically vulnerable and undersized: “How do you replace a sewer in a lake? What are the alternatives? How do you minimize public impact?” The solution was an innovative replacement and rehabilitation project, completed in May 2011 after a 10-year planning, design, and construction process. Total cost for the program was $95 million to replace and rehabilitate a deteriorating interceptor with a system designed to last at least 100 years. An underwater buoyant gravity sewer design was utilized in lieu of a traditional land-based open-cut sewer system, which would have involved 6 new pump stations, 75 percent more pipe, disruption to residential streets, and significant rock excavation. The completed Lake Oswego Interceptor Sewer (LOIS) system includes a 10,000–foot-long underwater buoyant system, consisting of 22- to 42-inch high density polyethylene pipe. This innovative solution was the lowest cost option, with little excavation work and the lowest carbon footprint. In-water reaches upstream and downstream of the buoyant, gravity sewer were rehabilitated utilizing trenchless technology to avoid difficult and disruptive open-cut replacement that would have required additional schedule during a compressed phase in which the lake was drained. Rehabilitation consisted of 19,000 feet of 8- to 36-inch diameter sewers with cured-in-place pipe (CIPP) and rehabilitation of over 50 manholes will extend the useful life of existing, in-lake, adequately sized and supported sewers that are part of the LOIS system.