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   Located in Black Canyon straddling the border between Nevada and Arizona in the southwestern region of the United States, Hoover Dam is named one of the Top 10 Construction Achievements of the 20th century. The dam, constructed between 1931 and 1936, was the largest of its kind at the time. Its construction was the result of a massive effort involving thousands of workers and cost over one hundred lives.        Since about 1900, the Black Canyon and nearby Boulder Canyon had been investigated for their potential to support a dam that would control floods, provide irrigation water, and produce hydroelectric power. In 1928, the US Congress authorized the project. The winning bid to build the dam was submitted by Six Companies, Inc. However, such a large concrete structure had never been built before, and some of the techniques were unproven. The extreme summer heat and lack of facilities near the site also presented tremendous difficulties. Nevertheless, Six Companies turned over the finished dam to the federal government on March 1, 1936, more than two years ahead of schedule.由文章可知dam的建造是由Six Companies, Inc承包，但是困難重重，dam的建造是幾千名工人努力的成果(當時也犧牲了100多條生命)，最終成功。