When pulses of light travel along a fibre optic strand, tiny reflections sometimes bounce back along that line. These reflections are affected by factors including temperature, vibrations or physical disturbance to the cable itself.

With this technology, it is even possible to work out the approximate size of a vessel passing above a subsea cable, as well as its location and, in some circumstances, its direction of travel. That could be correlated with satellite imagery, or even automatic identification system (AIS) records, which most ships broadcast at all times.

Mr Heiden argues that cables installed solely for the purpose of monitoring marine activity could be especially useful – one might place such listening cables, say, 100km from a vital port, or in the vicinity of a key gas pipeline or telecommunications cable, rather than within those assets themselves.