Fifty feet below the platform of the Powell Street BART Station sits the starting point for one of the largest water recycling projects in San Francisco — one that’s transforming dirty groundwater into clean steam heat for hundreds of downtown buildings. In the process, it’s saving tens of millions of gallons of drinking water annually.

For decades, BART officials treated the naturally percolating groundwater that pools beneath the BART stop as a nuisance and a potential flooding risk. After seeping into an underground cistern, millions of gallons of water a month was pumped into the city’s sewer system.