Every warship generates a magnetic field. It comes from the steel hull, the engines, the accumulated magnetism of thousands of miles at sea. The crew cannot see it. Neither can an observer on shore. But a magnetic sea mine — or a submarine detection system — can detect it from a distance.
During World War II, German magnetic mines nearly shut down British coastal shipping before the British worked out how to neutralize them. The solution was degaussing: running electrical cables around a ship's hull and passing a carefully calibrated current through them to counteract the vessel's magnetic field. The technique worked. The Allies refined it throughout the war.
The modern version of this process for submarines is called magnetic silencing, and a facility purpose-built for it operates at Ballast Point, inside the naval security perimeter. The underwater portion — a sensor array that measures a vessel's magnetic signature as it passes overhead — lies in the channel that submarines transit when departing the base for the open Pacific.
The process involves running large electrical currents through the submarine's hull — the facility at Point Loma uses approximately 4,000 amps — to precisely cancel the accumulated magnetic field. The submarine moves slowly over the underwater range, the sensors take readings, the current is adjusted, and the process repeats until the magnetic signature falls within acceptable limits.
A nuclear fast-attack submarine's primary advantage is invisibility. It runs silently, operates deep, and travels without broadcasting its position. A detectable magnetic anomaly is a breach in that invisibility — one that can be exploited by aircraft equipped with magnetic anomaly detectors, or by mines laid in transit lanes.
Degaussing does not make a submarine magnetically invisible in the way that a stealth aircraft is radar-invisible. It reduces the magnetic signature to a level that makes detection significantly harder, particularly by passive systems. A submarine that has been properly degaussed is harder to localize from the air or from a sea-bottom sensor array than one that has not.
From the water, the Ballast Point facility looks like an unremarkable naval pier with some overhead cable infrastructure. Nothing about it signals what it does. The underwater sensor array is invisible from the surface. Submarines passing over it from dock to open Pacific are indistinguishable, to an outside observer, from any other vessel transiting the channel.
Then they submerge, and the Pacific closes around them, and the signature that was already nearly gone disappears entirely.
Sources: Wikipedia, "Degaussing"; RQ Construction project page, Magnetic Silencing Facility; U.S. Navy Fact Sheet (via Time), Submarine Drive-In Magnetic Silencing Facility.
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