"Getting this crossing into place is important," said J. Richard Capka, chief executive officer of the Turnpike Authority, the agency overseeing the Artery-tunnel work. "The engineering part of the project is behind us."

Over 12 days, marine engineers maneuvered two battleship-sized concrete boxes into place in the Fort Point Channel -- a mere five feet over the MBTA's Red Line tunnels, which on a typical workday carry 85,000 commuters.

"This has been planned like a NASA shuttle launch," said Capka.

The channel section of the Big Dig is scheduled to open in August 2002 and will link the Massachusetts Turnpike with the Ted Williams Tunnel.

The Red Line was shut down for only three hours during the delicate operation. But that precautionary step was one of the smallest hurdles in five years of preparation to protect the commuting public and the T's vital subway tunnels and stations from a potentially catastrophic flood.

"If they breach that tunnel," MBTA general manager Robert H. Prince Jr. said last Saturday, hours before the second tunnel box was successfully steered in place, "I'm putting my papers in" to retire.

For years, MBTA and Big Dig officials have had pulse-racing "what if" conversations about the consequences of a gash in one of the concrete Red Line tunnels running underneath the channel.

Four years ago, at a local transportation engineers' gathering, officials outlined detailed measures they were already putting in place in case something went awry.

"The Red Line is the most important transportation access into the city from the South Shore," said the MBTA's Anne Y. Herzenberg, then chief of Red Line operations. It carries more people at rush hour than the Southeast Expressway, amounting to about 14,000 passengers an hour.

Even more important, however, is the fact that the Red Line's northbound and southbound tunnels, buried in the soil of the Fort Point Channel, connect to the north with South Station and other subway lines. A worst-case scenario: Water in the Red Line could mean a flood to the busy Park Street Station and into the Orange and Green line tunnels.

"We talked about closing the tunnel, in case of a mishap," said Herzenberg. "But we realized it just wasn't feasible to close down the Red Line on a workday."

So dredging operations were carried out at night, when no passengers were at risk. Twenty-two of the 26 feet of the dirt and clay bottom of the channel covering the Red Line was excavated in preparation for setting the tunnel boxes in place.

The two boxes lowered into place last month -- weighing 85,000 tons and averaging 380 feet in length -- now rest above the top of the two Red Line tunnels.

The new eastbound and westbound lanes of Interstate 90 are held up by scores of concrete columns 6 feet in diameter, extending deep into bedrock.

Mike Bertoulin, who oversaw the channel work, said immersing precast concrete-box sections in water is the first time such technology has been used in the United States.

Joe Allegro, director of Big Dig construction, outlined the extraordinary steps taken to contain damage to the Red Line from any potential accident:

Huge sheets of steel were driven into the bottom of the channel to protect the sides of the Red Line tunnels.

Electronic meters and mechanical devices were placed inside the tunnels to detect the slightest movement or cracks.

Buoys marked the location of the Red Line tunnels, and anchoring vessels being used in the Big Dig work were kept at least 10 feet from the tunnels.

Bulkhead doors were built inside the tunnels to be closed off in case of flooding.

A guidance system was installed on the bucket of excavating equipment used near the tunnels.

Big Dig managers have two more tunnel sections to place in the next couple of weeks, but those will be submerged east of the Red Line, in less sensitive territory.

"This project is the most exciting thing for a civil engineer," said John Bales, an engineer involved in the Fort Point Channel work. "This is paradise."