Golden Gate National Recreational Area - Mines and Submarine Defenses (U.S. National Park Service)

If the big guns failed to stop an enemy vessel far offshore, the next line of defense was three minefields containing over 600 underwater mines outside the Golden Gate. An antisubmarine net located inside the Gate would prevent any submarine that penetrated the minefield from entering the bay.

PARC, Golden Gate National Recreation Area

Four mineplanters, like the Spurgin shown here, placed mines outside the Golden Gate. They were supported by a flotilla of smaller craft, including many commandeered crab fishing boats.

PARC, Golden Gate National Recreation Area

A mine containing 3000 pounds of TNT is detonated during a test.

PARC, Golden Gate National Recreation Area

Early in the war, buoyant mines were anchored 15 feet below the surface and could be detonated from an onshore control station. "Ground" mines (above), which ships could not foul, later replaced the buoyant mines.

photo of Alcatraz prisoner working on antisubmarine buoy

PARC, Golden Gate National Recreation Area

The antisubmarine net that protected the bay was made and serviced at the Tiburon Naval Net Depot and at Alcatraz prison (above). Navy ships would open and close the net for friendly vessels. Here, an Alcatraz prisoner is working on an antisubmarine buoy.