When Congress authorized the construction of a massive dam on the Colorado River in 1928, America had never before seen such a massive undertaking of resources and labor. The Nevada town of Boulder City was developed for the sole purpose of housing the thousands of workers who were to build what would become the Hoover Dam. When complete, the nearly 730-foot-tall structure had used 5.9 million barrels of cement over the 27 years of construction.

The dam was built to divert the Colorado and control flooding by burrowing four massive, 30-foot-diameter tunnels into the walls of Black Canyon, two in Nevada and two in Arizona. Today, nearly 80 years after it began operating, the dam generates about 4 billion kilowatt-hours of hydroelectric power each year, enough to power the lives of 1.3 million people.

This National Historic Landmark has offered guided tours since 1937, drawing one million visitors each year. For $30, individuals can take an elevator 530 feet down through the rock wall of the Black Canyon for a tour through one of the dam’s four tunnels. The metal behemoths are capable of moving 90,000 gallons of water each second from Lake Mead to the dam’s hydroelectric generators.

Back above ground, the power plant balcony offers a panoramic view of the 650-foot-long Nevada wing of the plant, as well as eight of the dam’s 17 generators. An exhibit gallery houses memorabilia from the dam’s 82-year history along with a walk-through model of a generator and a detailed diorama of the entire dam.