BOSTON -- Big Dig workers will finish a huge phase of construction Thursday when the last portions of a tunnel are put into place in South Boston.

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NewsCenter 5's Gail Huff reported that crews arrived at the crack of dawn prepared to move two tunnel tubes through the channel into place.

Using ropes and pulleys, the operation will take 6 to 12 hours to finish.

The 51,000-ton traffic tube was built in a casting basin. Now completed, it must be moved through the water to its final resting place.

Eventually it will help complete the connection between the Massachusetts Turnpike and the Ted Williams Tunnel.

"We'll inch it over and pull it through," Big Dig Project Manager Mike Bertoulin said. "There's only a 3-foot of clearance, so every time we pull it a foot, we have to change its angle to just barely squeeze it through."

With the help of computers, experts will monitor the work that is going on underwater.

"They know on a computer screen where they are within a couple of centimeters of plan," Bertoulin said. " They'll take it down and set it -- almost hover it like a submarine would hover at a certain depth -- and then the tide will take it down the rest of the way and let it set on its temporary jacks."

Officials are hoping to have both tubes in place by sunset.