Eighty-eight concrete columns rise 80 feet from the floor here, in a cavernous tomb-like structure minutes south of San Francisco on Interstate 280.
Workers bang hammers and scurry around by the light of flood lamps ,making sure the structure can be watertight. A rectangle of sunshine beaming from the hatch far above is the only reminder that it's a bright day outside.
This is the most complicated, most involved -- and most impressive -- part of the $278 million remodel of the Harry Tracy Water Treatment Plant, near San Andreas Reservoir just over the hill from San Bruno.
And if all goes according to plan, these details of this crown jewel will never be seen again.
The columns support the 11 million-gallon treated-water reservoir under construction for the station that prepares about 15 percent of San Francisco's drinking water.
The rising towers give the reservoir's interior the impression of a tomb. Along the reservoir's base circumference is a layer of foam, through which run thick steel cables reaching to the reservoir's top.
The foam separates the reservoir's base from its sides, which can shake and shimmy as much as needed in an earthquake without breaking, even with 11 million gallons of water sloshing around inside.
That's good. Because when the next Big One hits, The City's drinking-water supply will rely on those columns and cables keeping the reservoir intact.
Harry Tracy is the closest project to The City that's a part of the Water Systems Improvement Project, the $4.6 billion rehab and overhaul of the network of pipes, tanks and aqueducts that carries San Francisco's freshwater supply.
Begun after voter approval in 2004, the WSIP is now about 80 percent complete, as is the work on Harry Tracy, one of the 86 WSIP projects, according to Daniel Wade, who oversees the WSIP for the San Francisco Public Utilities Commission.
Right now, Harry Tracy treats between 10 million and 20 million gallons of water a day from the Peninsula watershed, filtering it for impurities and applying chemicals to kill bacteria before it is delivered to San Francisco or other SFPUC customers.
Once the upgrade is completed this year or in early 2015, Harry Tracy will be able to treat up to 140 million gallons of water a day -- enough to supply The City twice over -- and will be able to do it within 24 hours of a major seismic event, and for up to 60 days at a stretch, the SFPUC claims.
That's something Harry Tracy can do now only for a "few hours," said Calvin Huey, the senior project manager.
If The City is ever cut off from its major freshwater supply from the Hetch Hetchy reservoir in Yosemite National Park, Harry Tracy is the backup emergency supply.
Harry Tracy's overhaul didn't quite go as planned: during initial surveying, an undiscovered fault line was discovered -- directly underneath the existing treatment reservoir. That meant building this new one, dug into the hillside below the treatment plant's filters.
Work started here in 2011, with work on the reservoir beginning in May 2012.
And all the while, the water still flowed north, with the plant shutting down only occasionally to allow workers access to places that would otherwise be flooded.
"It's like flying an airplane while fixing it," Wade said -- an airplane that you ride every day, for the foreseeable future.