Recent transplants to Boston could be forgiven for thinking we have but one harbor tunnel connecting downtown with East Boston. The twinned Sumner and Callahan tunnels, after all, begin and end in the same place on both sides of the harbor, a single entity for the casual observer. They were in fact built decades apart, the Sumner in 1934, and the Callahan in 1961. With increasing volume, a single tunnel with traffic moving on one lane in each direction proved dangerous and inadequate, and the Turnpike Authority opened the Callahan on Nov. 11, 1961. Workers toiled 24 hours a day, six days a week crafting 14,500 tons of steel into the nearly mile-long tunnel. - Lane Turner and Lisa Tuite.

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  Bob Backoff/Globe Staff

  Feb. 27, 1960: This is a view of the front end of the tunnel shield. A total of 134,000 cubic yards of material had to be excavated to complete the bore under the harbor. Sandhogs, from Boston Local 88, working 24 hours a day, six days a week, moved the excavation shield ahead an average of 16 feet per day.
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  Edward Jenner/Globe Staff

  Oct. 27, 1960: The movable shield shown here bit into the mud while at the same time provided protection against a cave-in. The rams that moved the shield and held back the mud were enormously powerful work units, and there were 28 of them. Each could exert a shove of 200 tons and move forward 48 inches. The shield operated like a giant biscuit cutter in removing the hard blue clay through which the tunnel was being dug.
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  Bob Dixon/Globe Staff

  March 3, 1961: This photo shows workmen as they exposed the pilot, 6-by-8-foot tunnel coming from Boston and the main tunnel coming from East Boston. The two tunnels met under North Street in Boston's North End. There was less than a half-inch variance from the plans on either side of the two tunnels. The foreman overseeing the breakthrough, on the right, was Ken Smith from Brighton.
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  The Boston Globe/ file 1961

  March 23, 1961: The steel ribs of the Lieutenant William F. Callahan Jr. tunnel stretched as far as the eye could see under Boston Harbor from East Boston to Boston. The 14,500 tons of special structural steel used to fabricate the basic lining of the tube came via rail to East Boston from Youngstown, Ohio. The massive corrugated series of curved steel plates when bolted together formed "rings." In the case of the new tunnel, concrete formed the secondary lining inside the rings. A total of 1,813 rings, each composed of 11 segments, were locked tightly into place.
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  Edward Jenner/Globe Staff

  Sept. 6, 1961: The 9 foot diameter of these powerful fans used for ventilation at the East Boston end of the new tunnel dwarfed project engineer Irving R. Huie (left) and Clyde H. Galbraith, coordinating field engineer for the equipment maker. The bank of three fans could move up to a million cubic feet of air per minute. The new Callahan tunnel opened on Nov. 11, 1961, the 40th anniversary of the birth of the late war hero. Only son of Massachusetts Turnpike Commissioner William F. Callahan, the lieutenant fell in action April 14, 1945. His death occurred during an attack in the mountains of northern Italy, a few weeks before World War II ended.