BOSTON -- A new stage in the task of cleaning up Boston Harbor began Wednesday.

NewsCenter 5's Pam Cross reported that a giant cutter head will cut a 2.1-mile tunnel under parts of South Boston. Eventually, the tunnel will hold thousands of gallons of runoff storm water that will be pumped to a treatment plant, eliminating any sewage onto harbor beaches.

Even before the tunnel is complete, efforts to clean the beaches received an A-plus.

"Today, the MWRA has closed 34 combined sewer overflow outlets, and overflow volumes have already been reduced by 75 percent," EPA Regional Administrator Robert Varney said.

"Too often, we have to close our beaches because the sewer system is overwhelmed by rain events and storms. This project today will solve that problem. It's the second half, really, of the cleanup of Boston Harbor that began with Deer Island," Secretary of Energy and Environmental Affairs Ian Bowles said.

The tunnel will be 19-feet wide, and it will cut through soft rock. The expected cost is $215 million and should be complete in four years.