for some of the other sections. As the foundations were completed, concrete placement of the main dam body could begin. For this purpose, a system of steel cable towers was erected to carry the steel concrete-pouring buckets. The tower setup comprised one main tower, standing high from its foundations and above the river, and seven movable auxiliary towers; cables were strung from the top of the main tower to each of the others. Using this system, construction crews were able to transport concrete from the mixing plant, which lay directly adjacent to the main tower at the end of the conveyor belt, to the rising structure of the dam faster and cheaper than any other method. The tower was not fully demolished, so it remains submerged behind the dam. On occasion, the lake level can drop low enough to expose it again. Concrete placement and river diversion Building of the dam's main concrete structure started in July 1940 after the cable systems had been completed and the mixing plant came on line. Steel buckets capable of carrying of concrete, weighing 16 tons when full, traveled back and forth along the lines. For three years, thousands of men labored building the dam's massive abutments, pouring concrete into large modular "blocks" square and deep. These blocks were shaped using wooden forms, which were removed when the concrete dried and rebuilt to handle the next block above it. The concrete was "vibrated" into place using specialized equipment, which filled in any accidental cavities and bubbles to ensure maximum density and strength. After the concrete dried, the surface was cleaned and smoothed by sandblasting. At the same time, the railroad that ran through the temporary tunnel on the west side of the river was rerouted. In December 1941, the Japanese attacked Pearl Harbor, prompting the United States to enter into World War II. With thousands of men enlisting in the armed forces, the Shasta Dam site soon had a severe labor shortage. During the war years, many of the people who worked at the dam were women and high school students, working on mostly "non-hazardous jobs". The dam played an important part in World War II even before its completion, supplying much-needed electricity to shipyards and aircraft factories in central California. However, some generators originally intended for Shasta ended up in the Grand Coulee Dam in northern Washington because of the enormous demand for