It is early days for the technology, at least in the West. But work done in Russia, whose navy has long been interested in alternatives to sonar, suggests the Debye effect can be turned into something quite potent. In 1990, two contributors to the Soviet military magazine Naval Collection wrote that “as a consequence of the great extent of the wake, it is easier to detect this anomaly than the magnetic anomaly due to the metallic hull of the submarine.” That suggests that a well-tuned Debye detector might be able to pick up a trail from several kilometres back and follow it to find the submarine. Russia’s claims in this area have long been regarded in the West as exaggerated. The new American interest suggests they might not have been.

This, and supercavitating torpedoes (and their focus on diesel-electrics rather than loud nuclear subs), gives them a distinct advantage, and has for a number of years.