MENLO PARK - The San Francisco Public Utilities Commission gave a guided tour today of the Bay Tunnel project, a planned $286 million, earthquake-resistant water pipeline and the first tunnel to be built beneath the floor of the San Francisco Bay.

Julie Labonte, director of the SFPUC's water system improvement program, and Jim Stephens, project manager of the Bay Tunnel project, took news reporters in an elevator down a 110-foot shaft to the western end of the 5-mile tunnel near the shore of the bay in Menlo Park.

Labonte, Stephens and journalists in safety vests and hard hats walked about 50 feet into the dark, damp 108-inch round steel pipeline, reinforced by more than 2 feet of cement to make it resistant to temblors and corrosion.

"This has been challenging," Labonte said. "This is a 5-mile tunnel that is being built without any intermediate shaft and it's under very high hydrostatic pressure because we're basically tunneling under the bay."

The Bay Tunnel, which is about 90 percent complete, is to be part of the 176-mile Hetch Hetchy Regional Water System that pipes fresh water from reservoirs outside Yosemite National Park to San Francisco, serving 2.5 million people.

The tunnel runs from Newark down to a maximum of 125 feet under the floor of the bay to the edge of Menlo Park, a mile south of the Dumbarton Bridge, according to Labonte.

To be completed by next summer, it is the first tunnel constructed under the bottom of the bay. BART's Transbay Tube was built on top of the bay's floor, Labonte said.

"A lot of people think that the BART tube is a tunnel, but it's not," Labonte said. "It's a tube that was laid into a trench, but this a true tunnel. We have a tunnel through bay mud here."

The tunnel is part of a series of 81 projects the SFPUC began 10 years ago after the agency conducted studies and found that a major earthquake in the Bay Area could "create a catastrophic failure of the Hetch Hetchy system which in turn could result in parts of our service area to be without water for up to two months," Labonte said.

San Francisco Bay Area sits on what is known as the "Ring of Fire" along the Pacific Ocean where 90 percent of the world's earthquakes occur and the U.S. Geological Survey predicted in 2008 that the Bay Area had a 63 percent chance of a major earthquake within 30 years, Labonte said.

"So, those probabilities are now so much higher," Labonte said.

The SFPUC purposefully organized the tour today to coincide with the 24th anniversary of the 1989 Loma Prieta earthquake in the Bay Area, according to Labonte said.

That quake killed 63 people, injured more than 3,700, caused about $10 billion in damage and the collapse of parts of Interstate 880 in Oakland and the San Francisco-Oakland Bay Bridge, according to the California Department of Conservation's website.

In light of the dangers posed by earthquakes, the SFPUC approved a $4.6 billion water improvement plan a decade ago to repair and replace aging sections of the water system to make them seismically safe, including building the Bay Pipeline, Labonte said.

So far, 62 of the 81 projects in the plan, across seven counties from California's Central Valley to downtown San Francisco, have been completed, she said.

The Bay Tunnel is considered a key part of a "lifeline" to deliver fresh water across the bay to SFPUC customers in San Francisco, San Mateo and Santa Clara counties even in the event of an earthquake, according to Labonte.

"It is the one that will survive a major earthquake," she said. "It is possible that other (pipeline) conduit could fail. But this one has been designed to withstand a maximum credible earthquake on all the major faults here in the Bay Area."

The project, to be completed by sometime next summer, is also meant to replace a pair of aging water pipes, built across the bay on wooden trestles in 1925 and 1932, still in use today, Labonte said.

To build the tunnel, the SFPUC ordered a giant earth boring machine, custom built in Japan, to dig a 15-foot-wide circular hole, starting 110 feet below ground level, from the Menlo Park side toward Newark.

Crews of miners at one point worked 10-hour shifts without leaving the tunnel and found that they were able to dig about 100 feet per day instead of the 50 feet builders had estimated, putting the project six months ahead of schedule, Labonte said.

The excavation started in August 2011 and the machine made it to the Newark side in January 2012.

Along the way, large hydraulic arms at the rear of the machine placed curved cement blocks to reinforce the tunnel every five feet, according to Labonte.

The machine installed a total of 31,400 cement blocks before the steel pipe, inserted in sections, was placed inside, Labonte said.

Pressurized water coming down via gravity on sloping pipelines from the Hetch Hetchy Reservoir near Yosemite will start flowing through the Bay Tunnel and into the existing system to San Francisco in late 2014, Labonte said.

"You have enough hydraulic head to push water down all the way to the city of San Francisco," she said.