Hoover Dam was born from fear of the Colorado River, which flooded in spring and dried to a trickle by late summer. After a canal to California’s newly branded Imperial Valley was breached, the river poured into a desert basin for two years and created the Salton Sea, and the Southern Pacific Railroad spent $3 million forcing it back. When the St. Francis Dam collapsed in March 1928 and killed up to 600 people, Congress demanded an arch gravity design on ultra conservative lines and passed the Boulder Canyon Project Act that December. By 1931, Depression-era workers and their families were crowding into a squatter camp called Ragtown, where temperatures reached 120 degrees. High scalers dangled from ropes with jackhammers and dynamite, and dipping cloth hats in tar led Six Companies Inc. to order hard hats for the site.

Superintendent Frank Crowe faced $3,000 a day in penalties, and when tunnel workers struck in August 1931 over a wage cut, he fired them all. Officially 112 people died, but 42 deaths in 140 degree diversion tunnels were labeled pneumonia, which workers said hid carbon monoxide poisoning. To stop the concrete from taking 125 years to cool, crews poured it in blocks chilled by 582 miles of pipe. Gordon Kaufmann gave the dam its Art Deco look, and Allen Tupper True drew on Navajo and Pueblo designs for the interiors. The dam starved the Colorado River Delta and now faces a megadrought that has pushed Lake Mead toward dead pool.

• Oscar J.W. Hansen’s terrazzo star map fixing the sky of September 30, 1935, for future astronomers
• Surveyor J.G. Tierney drowning in 1922, and his son Patrick Tierney dying on the project 13 years later to the day
• How Harold Ickes renamed it Boulder Dam in 1933 until Congress restored Hoover’s name in 1947
• Cavitation during the 1983 floods that blasted chunks of concrete from the spillway tunnels
• Cold water released from the bottom of Lake Mead pushing the humpback chub and other native fish toward extinction