SUNOL VALLEY (KPIX 5) — In serene Sunol Valley, a monumental feat of engineering is just wrapping up.

Daniel Wade’s job is to make sure after a major quake, you still get water from the Hetch Hetchy Reservoir.

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“As a result of this project, the Bay Area is safer than it was even just a few days ago in the event of a major earthquake,” Wade said.

But Northern California’s terrain makes that a challenge.

“Our water system crosses all the three major earthquake fault lines in the Bay Area,” Wade said.

The Calaveras, the Hayward and the San Andreas faults are ticking time bombs. Making matters worse, all that Hetch Hetchy drinking water flows through one, very old tunnel in Sunol — a tunnel crews haven’t been able to inspect in nearly 50 years.

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“That tunnel hasn’t been taken out of service since 1966 because it’s so critical to the service of our customers,” Wade said.

But that’s changing. The San Francisco Public Utilities Commission spent $340 million to build a brand new second tunnel. David Tsztoo has headed the tunnel project for four anxious years.

“It’s like waiting for a baby to be born,” Tsztoo said. “You’re tense, and there’s a sense of anxiety because you don’t know what to expect mining a tunnel like this.”

Tsztoo says it had hiccups like groundwater flooding. But the new tunnel is done and seismically safe. Lined with steel and reinforced concrete, it’s able to stand up to a magnitude 7 earthquake.

“This tunnel is critically important to make sure we have a seismically reliable water system for decades to come,” Wade said.

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Crews will spend the next several weeks inspecting the old tunnel to see if it is still safe to use.