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.