"I think we're within of an inch [of perfection] all the way around," said bargemaster James "Big Jim" Harris, who coordinated the delicate operation of sinking the 365-by-160-by-27-foot section into place from a trailer-like room mounted atop the section.

"That's a beautiful piece of engineering," said Frederick Salvucci, former state secretary of transportation, who planned the Big Dig in the 1980s. Engineers agreed that yesterday's maneuver was probably the most complicated part of the $10.8 billion Central Artery-Ted Williams Tunnel project,

The vast section of underwater highway was placed more quickly than expected. It was winched with heavy blue, polypropylene ropes from its berth east of the Fort Point Channel, where it and similar sections had been constructed over the last several years.

It took 9 1/2 hours to place the section, which already supports foundations for a ventilation building and columns that will support a new Dorchester Avenue Bridge.

The operation was started at high tide so there would be maximum clearance between the massive structure and the two old MBTA Red Line tubes that run under most of the channel's length. The section, which workers began moving after dark Sunday, went down with the tide yesterday morning -- assisted by thousands of gallons of water pumped into four ballast tanks -- and landed on four powerful corner jacks just before 6 a.m.

Michael Bertoulin, South Boston area manager for Bechtel/Parsons Brinckerhoff, the state's consultant for the project, said conditions were nearly perfect.

"There wasn't a breeze. The water was dead calm," he said. He watched the all-night operation from behind the Gillette building and celebrated with the crews as the sun rose.

Most of the monitoring was done by radar and other high-tech apparatus, so engineering teams were only slightly hindered by the darkness.

"We had some luck," said Harris. Had there been problems, crews could have held the box in place short of its destination and waited for the next high tide. But it progessed so smoothly -- 20 feet a minute at one point -- that they were able to move it over the entire 790 feet and set it down all in one cycle.

The section rests on 41 concrete shafts that extend into bedrock. In the next week, divers will cement the shafts to the underside of the tunnel box.

This massive piece is one of six that will be placed across the channel before 2002, providing access from the Southeast Expressway and the Massachusetts Turnpike to the Ted Williams Tunnel. They were designed by Gannett Fleming of Harrisburg, Pa., and Hyder Consulting Inc., based in England.

The tunnel sections are being built by Modern Continental Construction Co. of Cambridge, a prominent Big Dig contractor whose owner, Les Marino, left a sick bed before dawn yesterday to witness the milestone.

"It couldn't have been more accurate than this," he said. "It was amazing."

This was the third precast tunnel job that John Bales of Bechel/Parsons has overseen. The other two were in England and Wales. He said this piece was the most complicated, with portions of a bridge and a building on top of the tunnel section. "It's got a lot going for it, this one," said Bales.