BOSTON -- It's been under construction for six years, but hardly anyone has heard about the other big dig, and even fewer have seen it.

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Watch Jorge Quiroga's Report

NewsCenter 5's Jorge Quiroga said that the Massachusetts Water Resources Authority is almost finished building a new aqueduct that will supply water to greater Boston.

From the air, all you see is a concrete shaft, 36 feet wide.

"This is main access point. All the work for tunnel begins here and ends here," project director Mike McBride said.

McBride deals with gritty engineering realities, not pulp fiction. The tunnel is 16 feet wide and 18 miles long. He's walked every foot of it.

"From an engineering point of view, it's just a very thrilling piece of work. It's a very big project. It's extremely complicated," McBride said.

Mined through solid bedrock, the tunnel runs under five communities, from Route 495 in Marlborough, Mass., to Rte.128 in Weston, Mass.

"There is all kinds of details between working in residential neighborhoods and mining under peoples houses and adjacent to wells and ponds and all the things that go with it. There is a lot to think about," McBride said.

Inside the dark, dank, narrow cylinder everything shipped in and out is ferried by train. For train conductor Bob Martone, it's just another day at the office.

"It's like any other job -- you just got a job to do and you just do it," Martone said.

A 10-minute train ride and a soggy walk later, we reached our destination. Twenty-four hours a day, five days a week, work crews pour concrete, forming a foot-thick envelope that effectively seals the bare-rock tunnel walls. Just to give you an idea of how big a job this is -- they been at this four years.

"Every day there are challenges and in many ways, this is like making sausage, it's kind of ugly, but when the product is done it will be great," MWRA Executive Director Fred Laskey said.

When the whole job is done, the tunnel will be completely filled with water, flowing at a rate of 250 million gallons a day towards Boston.

It's a badly needed replacement for an aging aqueduct with 35 major leaks, and with a $1.6 billion price tag, it's on schedule and under budget.

"In light of some of the other projects, we are very cautious. We don't want to look like we are boasting, but knock on wood, it's going very well," Laskey said.

The tunnel is expected to open in min-2003.