a debate is emerging between the San Francisco Public Utilities Commission (PUC) and the Bay Area Water Supply and Conservation Agency over the potential disaster if the 89-year-old Mountain Tunnel, which supplies 2.6 million Bay Area homes and businesses with water, collapses. The PUC acknowledges that the tunnel does have a chance of "catastrophic collapse," which would require repairs costing $100 million or total replacement, costing up to $630 million. But the PUC’s 10-year-old 4.6 billion water system improvement program did not include the Mountain Tunnel in its plans. ... The risk right now is that the tunnel lining could...