DigIndy project, a $2 billion tunnel-system project to improve local waterways, is more than half-completed and is running $400 million under budget, Citizens Energy Group announced Monday.

The 28-mile network of tunnels is being built 250 feet below ground to eliminate sewer overflows into area waterways by 2025. The tunnels are 18 feet in diameter.

When complete in 2025, DigIndy will prevent up to 99 percent of sewer overflows now affecting area waterways when the city receives as little as a quarter-inch of rain, the utility said.

Since opening the first 10 miles of the system on Dec. 29, 2017, DigIndy has prevented more than 500 million gallons of sewage from overflowing into the White River and Eagle Creek across southern Marion County.

The project will satisfy a federal mandate to clean up Indy’s antiquated sewer system. In 2006, the U.S. Environmental Protection Agency and the U.S. Department of Justice reached a consent agreement with the city to eliminate 97 percent of combined sewer overflow.

“DigIndy is now functioning just as it was designed. Keeping 500 million gallons of sewage out of area waterways over the past nine months is a tremendous accomplishment resulting from years of careful planning,” said Jeffrey Harrison, CEO of Citizens Energy, in written remarks.

The utility held a press conference Monday with Mayor Joe Hogsett at the Fall Creek Tunnel drop shaft project site on Sutherland Avenue, between College and Guilford avenues. Hogsett called DigIndy a “transformational project for Indianapolis” that could spark economic and neighborhood redevelopment, in addition to keeping sewage out of local waterways.

The fact that the project is running under budget is consistent with updates from Citizens in recent years. In 2015, the utility said the project was running two years ahead of schedule and $330 million under budget.

The Indianapolis Department of Public of Works and IndyGo are working with Citizens to coordinate project schedules such as bridge replacements and the Red Line construction.

In addition to building the tunnel system 250 feet beneath the city, Citizens is constructing new consolidated sewers and drop shafts that will convey sewer overflows to the tunnel. Over the next seven years, the utility said, residents will notice large cranes in areas of the city where drop shafts are being constructed. The utility has already more than doubled the capacity of its two advanced wastewater treatment plants to process volumes captured by the tunnel system.