City officials are seeking bidders for the first phase of Indianapolis' largest-ever public works project, an underground tunnel system equipped to store millions of gallons of raw sewage and prevent the excrement from flowing into local waterways.

The project, part of a 2010 settlement with the U.S. Environmental Protection Agency, calls for a system of five wastewater storage tunnels to be built by 2025 at a total cost of more than $1 billion.

The first phase, a so-called Deep Rock Tunnel Connector, would be built 250 feet below ground and stretch seven miles north from the Southport Advanced Wastewater Treatment Plant on the southwest side. It would hold up to 54 million gallons of raw sewage, and cost up to $300 million to build.

The Department of Public Works plans to start seeking bids for the work on May 16, and is set to open bids July 19. Construction is scheduled to begin in late 2011 or early 2012.

The other four storage tunnels will run along White River, Fall Creek, Pleasant Run and Pogues Run. The city's agreement with the EPA requires the system be operational by 2025.

City and federal officials reached a deal in 2010 that paved the way for the tunnel system. The agreement includes an accelerated construction schedule for the city's efforts to reduce sewage overflows from systems that carry both storm runoff and sanitary waste.

The new plan is expected to reduce overflows from about 7.8 billion gallons to about 414 million gallons per year.