Mines and Submarine Defenses
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.
Did You Know?
It takes about 1,000 years of radiolarian “rain” to produce a layer of ribbon chert just one millimeter thick.