SUBMARINES rely on stealth to do their jobs, whether that is sinking enemy ships or hiding nuclear-tipped missiles beneath the ocean. The traditional way of hunting them is with sonar. Modern sonar is extremely sensitive. But modern submarines are very quiet, and neither side has gained a definitive upper hand.

There are other options. Submarine-spotting aircraft carry “magnetic anomaly detectors” (MAD) which pick up disturbances in the Earth’s magnetic field caused by a submarine’s metal hull. Those disturbances are tiny, which means MAD is only useful at ranges of a few hundred metres.

There may, though, be a better way. Thanks to something called the Debye effect, it might be possible to hunt submarines using the magnetic signatures of their wakes. Seawater is salty, full of ions of sodium and chlorine. Because those ions have different masses, any nudge—such as a passing submarine—moves some farther than others. Each ion carries an electric charge, and the movement of those charges produces a magnetic field.

The Debye effect has been known since 1933, but its effects were thought to be tiny. The American navy set out to explore it nonetheless in 2009, giving research grants to three firms to check whether it could be used for submarine detection. One, Cortana Corporation of Falls Church, Virginia, found a significant effect. Cortana was given a second grant in 2011 to continue the work, which was expected to produce a sensor which could be deployed from a ship. Since then the navy has continued to award Cortana grants for hush-hush jobs.

Neither Cortana nor the navy will discuss exactly what they are up to. But it is likely that the technique can only detect certain submarine movements in some situations. Submarines produce many different types of wake. As well as the familiar V-shaped wake they leave underwater disturbances known as “internal waves”, flat swirls called “pancake eddies” and miniature vortices which spin off from fins and control surfaces. These all depend not only on speed and depth but also on the submarine’s hydrodynamics (the underwater version of aerodynamics).

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.

Things are likely to get easier, too: a new generation of high-tech magnetic sensors based on machines called SQUIDs—“superconducting quantum interference devices”—should be more sensitive than existing ones. Both America and Britain are in the midst of replacing their present generation of nuclear-armed submarines. The new boats will be some of the quietest ever built. But if their wakes give them away, that may not matter.

Correction (November 22nd): A previous version of this story referred to a place called "Church Falls, Virginia". Apologies to residents of, and all others who love the fine city of Falls Church in that state.