High above the floor  of Nevada’s Black Canyon, a group of men repelled down the cliff face. Suddenly, a man stumbled  over the canyon rim. He plummeted past the men,  too fast for them to grab— when another climber swung  out from the rocks and carried him to safety. Once they’d repelled  the engineer up the cliff, his savior swung back to business as usual— just another day in the construction  of the Hoover Dam. In the early 20th century, the United States had expanded  from coast to coast, but many cities in the arid southwest still lacked reliable water sources. While the Colorado River had been diverted into a series of canals, its erratic flow and frequent floods made it unreliable for agriculture. At the same time, the region’s growing cities and industries continually needed more energy. So in 1922, the Bureau of Reclamation decided to solve all these problems in one fell swoop by building  a massive hydroelectric dam. Ensuring water rights for the project required unprecedented cooperation between all seven states along the river. But once local governments agreed, the Bureau determined that Black Canyon’s narrow gorge was best suited for an arch-gravity dam. This design curves upstream, using the force of oncoming water to push the structure into its supports— in this case, the canyon walls. Arch dams have been built  since ancient times, but never on such a massive scale. When complete, the structure would be 221 by 379 meters. It would take an enormous labor force to build a dam this massive. And when the Great Depression struck  only a year after the project’s approval, thousands of families looking for work began flocking to the site. The earliest arrivals lived in a makeshift encampment called Ragtown. With no infrastructure, scarce supplies, and little protection from the elements, several residents died of heatstroke. So to better house workers, the government designed and built the still thriving Boulder City, complete with numerous amenities, including a state-of-the-art hospital. Before construction on the dam  could begin, the entire river had to be rerouted. Building these diversion tunnels required blasting into each side of the canyon to create 17 meter tall tubes. With a combined length of almost 5 kilometers, these tunnels could divert roughly  5,600 cubic meters of water per second. However, workers faced grueling conditions to finish these on time, and dozens succumbed  to the scorching heat. By the fall of 1932, the river had been successfully redirected. But the most dangerous work was still ahead. Over 1 million cubic meters  of loose rock needed to be cleared from the canyon walls. So foremen brought in high scalers  who descended the cliffs on ropes, removing rubble with jackhammers and dynamite. From circus acrobats and former sailors, to local Apache, these death-defying individuals performed some of the most dangerous— and glamorous— work on site. After the walls were cleared,  it was finally time to build the dam. This required pouring over 6.6 million  tons of concrete— enough to pave a road  across the entire US. But so much concrete poured at once would take far too long to cool and harden. So to speed this process up, concrete was poured in interlocking blocks of varying sizes, each containing steel pipes flowing with cold water. By 1935, most of the structure had been completed two years ahead of schedule. On February 1st, the diversion tunnels opened to fill the dam’s reservoir, capable of holding twice  the Colorado River’s annual flow. Upon completion, the dam was the tallest manmade structure in the world. However, the speed of its construction came at the cost of worker safety. While this project employed roughly 21,000 Americans during a major economic crisis, over 100 people died during construction. Creating the reservoir also destroyed communities like St. Thomas and devastated the Colorado River  ecosystem. Today, the Hoover Dam generates  over 4 billion kilowatt-hours annually, providing electricity  for over 1.3 million people. But the consequences of its construction continue to cast a shadow over this monumental structure.