The source of water for 2.6 million customers has been cut off, on purpose. Workers are taking advantage of progress on another project to make repairs to a tunnel built nearly a century ago.It is loud, cold and wet inside this stretch of aging pipeline in the high Sierra, which opened in 1925.The mountain tunnel near Yosemite is an integral part of the Hetch Hetchy water system and operators feared it was on the verge of collapse.The cracks may look awful, but things are not as bad as engineers expected. "While they look big, structurally the tunnel is perfectly sound," said structural engineer Glenn Boyce.He has been walking this tunnel, inch by inch, looking for crack, or defects. "I've gone through on every thousand foot interval," said Boyce. "Some of them have about twenty defects which basically average out to about one every 50 feet."The San Francisco Public Utilities Commission has known since the late 80's that this tunnel needed repairs, but its location made it difficult to do a detailed inspection.In fact, the PUC had to drain an entire reservoir to get to it. "We would be underwater right now if this reservoir was filled," said San Francisco Public Utilities Commission Director Harlan Kelly.Typically workers get a week or two every 20 years to inspect and repair the tunnel. This time, they will get an unprecedented 90 days to make repairs.They are also enlarging two access points, so it will be easier to make future repairs. "We feel that the tunnel is in really good shape," said Kelly.This tunnel turns out to be much stronger and thicker than anyone expected. "On the plans it says a minimum of six inches," said Kelly. "But when we did core samples, we have 18 inches."The Hetch Hetchy water system was an engineering marvel when it was built. The system of pipes, dams, and reservoirs carries water 167 miles from Yosemite National Park, across the Central Valley, and into the South Bay before going up the Peninsula to San Francisco.Despite its age, it continues to impress. "It's just amazing the workmanship that they put in place 90 years ago, to have it stand up to the test of time," said Kelly.Work on the tunnel will wrap up next week and the water will be turned back on March 7, 2017.