Thousands of workers toiled 24 hours a day to build what was then the largest hydroelectricity facility in the world. The dam stopped up the Colorado River, creating Lake Mead, the biggest reservoir in the United States. At its height, the lake surface sits over 1,200 feet (365 meters) above sea level. But after more than two decades of drought it is now less than 1,050 feet -- the lowest since the lake was filled, and falling about a foot a week.
