Enlarge / “Did you feel that?” (credit: OIST)

There are enough seismometers around these days to detect and locate nearly all earthquakes on land, except the most minuscule ones. The seafloor is another story. It’s expensive and difficult to maintain seismometers at the bottom of the ocean, so our coverage is pretty sparse. Earthquakes smaller than a magnitude four won’t register in many places—and “many places” in the ocean means a large portion of the planet.

A group of researchers led by Giuseppe Marra of the UK’s National Physical Laboratory accidentally stumbled on an inexpensive way to fill in many of the gaps. They were working on advanced fiber optic links capable of connecting things like next-gen atomic clocks around Europe. At this level of sensitivity, any vibrations near the cable introduce noise in the signal, which is generally a nuisance to be overcome.

But while running an experiment on a UK cable in August 2016, they realized the noise in the line was coming from a magnitude 6.0 earthquake in Italy. As the seismic waves pass through the fiber, the resulting distortion slightly delays the signal, causing a measurable shift in the peaks and troughs of the oscillating light wave.