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For most of the Cold War it was a deep, dark secret. Although completely declassified in 1991, even today, few really understand what SOSUS was (and is) and what it could (and can) really do.

SOSUS is an acronym for SOund SUrveillance System. In 1949 Congress first authorized money for what eventually became a world-spanning ocean surveillance system. That system, mothballed after the end of the Cold War, is coming back to life as a major weapon to fight the terrorist threat on the high seas.

Here is how the system works:

Sound generated by an object in waters deeper than 100 fathoms radiates in all directions. Sound near the surface is absorbed and scattered quickly by surface wave action. Sound that radiates downward, however, eventually is bent back toward the surface by the increasing pressure of the water column. Typically, this sound reaches the surface about 30 nautical miles from the source, forming a ring of sound about 3 nautical miles wide centered on the source. This sound reflects again, and eventually arrives back at the surface 30 nautical miles further out in a 6-mile-wide ring, and again at 90 nautical miles in a 12-mile-wide ring, and at 120 nautical miles in a 24-mile-wide ring. After that, the entire surface of the ocean contains sound from this original source.

Obviously, the level of this sound is a tiny fraction of the ambient noise level, way below the background noise generated by the ocean and everything it contains. Nevertheless, in principle any sound-producing object, whether ship, submarine or marine animal, projects its sound over the entire world ocean.

What SOSUS did was nothing less than enable the U.S. Navy to identify and track all of these sounds. From a practical engineering point of view, two problems must be solved in order to do this. First, you must find a way to filter out and identify the sounds produced by one ship, sounds that are orders of magnitude below the ambient sound level. Remember, this task was accomplished way before the advent of digital technology. It was done so well that, when I was a young Navy Sonar Technician in the early 1960s, I could take a chart produced by a SOSUS array and identify a specific vessel by name and hull number, often thousands of miles distant from the receiving array.

The second task is to locate the source of that sound, to pinpoint it in the vastness of the world's oceans. This is why we placed arrays consisting of over 1,000 hydrophones and more than 30,000 miles of interconnecting

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