SAN FRANCISCO (KPIX 5) – On Sunday, the corner of Fremont and Mission in San Francisco’s South of Market became a construction site of epic proportions.

“This was one of the largest pours in San Francisco history,” said Rob Kain, construction project director of Boston Properties. “There were over 1,300 truckloads of concrete that were brought to the site yesterday in 18 hours.”

Construction crews poured a 14-foot thick slab, as tall as a double-decker bus, reinforced with steel rebar the size of a man’s fist. It will become the foundation of a building that will one day dwarf San Francisco’s skyline.

The building is called the Salesforce Tower. When it’s completed, it will be nearly 300 feet higher than The City’s current tallest resident, the Transamerica Pyramid.

Right now, it’s just a hole in the ground, but eventually the tower will be an iconic anchor to the future Transbay Terminal complex.

“This was a significant milestone for us,” said Salesforce Tower senior project manager Michael Tymoff. “It allows us to start going vertical with the project.”

Once that starts, it will happen surprisingly fast. “By January, we’ll be above the ground and by the end of 2016 we’ll have topped off at 1,070 feet, the tallest building west of Chicago,” Kain said.

To get an idea of that scale, the new structure will be nearly twice as high as the Millennium Tower that sits just across the street. But you can’t make a building that tall without a solid footing.

“Yeah, this is probably the more challenging part of the project, is getting the foundation complete. So, we’re real happy to be done with that,” Kain said.

Salesforce is the tower’s primary tenant. The developer of cloud-based business software is expected to move in sometime in 2017.