Forty million people depend on one river in the American desert, and the reservoir holding it back is now just thirty feet from failure. Thirty feet until the turbines inside Glen Canyon Dam go silent for roughly five million people. Below that waits something worse, the point where the Colorado River can no longer pass through the dam at all. The federal government's own forecasters say the first line could be crossed as early as next spring. And the reason it got this close has been hiding in plain sight for more than a century.