When finished Salesforce Tower will be the tallest building in San Francisco. For now, it’s a big hole in the ground. And at the bottom of that hole is a new, massive concrete slab—14 feet thick, spread nearly an acre in breadth, and ready to support 1,070 feet of glass, steel ... and a lot more concrete. How much concrete? About 12,000 cubic yards, enough to fill nearly four Olympic swimming pools. That concrete began as (roughly) 7,900,000 pounds of cement, 9,000 cubic yards of sand and gravel, and 54,000 gallons of water at several mixing plants around the Bay Area. Over the course of 18 hours - beginning just before midnight on November 7 - 1,300 mixing trucks ferried the stuff to the site in downtown San Francisco. Then, two at a time, they dumped their contents into one of seven pumping trucks.