This 100,000-year-old bay mud came up from the bottom of the hole that will one day house the lower levels of the new transit center.
Workers have spent the last three years demolishing and digging down through layers of history. Now it's time to start building up.
"We've gone back through a time when ships moored here and bat rays and leopard sharks swam here," said Michael Theriault with the Building and Trades Council. "We've gone back to a scene when woolly mammoths grazed here."
Sixty five feet below street level, concrete is being poured on a massive scale. Beginning now, four San Francisco city blocks will be covered with concrete five feet deep. It is the foundation of the Transbay Transit Center. The old Transbay building was hardly more than a bus terminal.
The transit center, as depicted in the Transbay Joint Powers animation, will be an entire multi-level facility with nearly a dozen transit agencies circulating in and out, including Caltrain and high speed rail.
"There will be 27,000 permanent jobs attached to this Transbay Transit Center," San Francisco Mayor Ed Lee said. "Ongoing, sustained."
The city got $400 million in federal funding for the Transbay center, and has parlayed that into private investment that is revitalizing the entire South of Market district around it.
"Six million square feet of new office space, 4,500 new housing, 1,200 of which are going to be permanently affordable," Mayor Lee said.
The transit center will include a new, 61-story high rise, housing businesses only. All around, private investors are building retail and housing.
"This is more than a bus terminal because you already have 19 buildings in construction because of this site," San Francisco Supervisor Jane Kim said. "These 19 buildings would not be coming up right now if not for the Transbay Terminal."
Target date for completion is 2017.