A giant custom-made mining machine the size of a six-car subway train emerged from the ground on the Dorchester border next to Carson beach yesterday. The machine took a year to build, but finished its 2.1 mile-long, 17-foot-in-diameter slog through sand, gravel and Boston blue clay all the way from Conley Terminal next to Castle Island in eleven months, over six months ahead of schedule.

"This a job you'll never hear about," said Massachusetts Water Resources Authority's chief engineer Charlie Button as the machine poked its way through a concrete slurry wall in a pit behind the State Police Barracks. "It's under budget and ahead of schedule."

The early finish took the price down from $180 million to $152 million, said Button.

From its position under Day Boulevard, the tunnel will collect runoff from 'combined sewer outflows' along the coast of South Boston. During large rain events, these CSOs combine storm water and sewage and spill into Boston Harbor. The new tunnel is designed to prevent that from happening in all but the 25-year storm.

Much to the chagrin of Corcoran Jennison Companies, which is building a retail shopping center with 350 rental apartments called Bayside on the Point next door, the spot where the boring machine is emerging will become an odor control facility.

The tunnel is the largest of the MWRA Combined Sewer Outflow (CSO) reduction projects of dozens mandated by federal and state courts.

The larger CSO effort is designed to improve the water quality of all the state's rivers, beaches and other bodies of water. The beginnings of it go all the way back to 1983, when the city of Quincy sued the Metropolitan District Commission (part of which became the MWRA) for the illegal discharge of untreated sewage into Boston Harbor. The Conservation Law Foundation filed a similar case in Federal Court six months later.

The end result of these two cases was a long-term remedial action enforced by the court and still in progress today. The court mandated an MWRA plan for some 35 CSO reduction projects across the state, 21 of which were completed by the end of last year.

As for the whole program, in 1998 the outfalls were discharging 3.2 billion gallons in a typical year. Ria Convery, an MWRA spokesperson, said the agency has closed 27 of those outfalls so far, and reduced about 2.7 billion gallons since 1988, around 81 percent. When completed in 2015, the agency hopes to have closed a total of 36 outfalls, and 93 percent of the remaining 500 million gallons of discharges will be treated before release.