Steve Richie, (left) assistant general manger of water, power and sewer for the city of San Francisco and project construction manager Robert Mues, inside adit 8/9 one of several access tunnels that lead to the Mountain Tunnel near Groveland, California,

Steve Richie, (left) assistant general manger of water, power and sewer for the city of San Francisco and project construction manager Robert Mues, inside adit 8/9 one of several access tunnels that lead to the Mountain Tunnel near Groveland, California, on Thurs. July 28, 2016. Mountain Tunnel, a key piece of the Hetch Hetchy water system is at risk of collapse, so this summer, the San Francisco Public Utilities Commission is preparing to repair the 19-mile-long tunnel just outside of Yosemite in a steep, hard-to-access wilderness area. (Photo by Michael Macor/San Francisco Chronicle via Getty Images)