Internet cables that crisscross the sea-floor could be used to detect earthquakes and tsunamis or monitor how climate change alters ocean currents.
These telecoms cables could be used as a giant array of deep-sea scientific sensors, the UK's National Physical Laboratory (NPL) and its partners say.
Scientists tested the technique on an optical-fibre link between the UK and Canada.
The research is published in Science Magazine.
Because installing permanent sensors to monitor the ocean floor is very costly, only a few exist globally, the scientists say.
"70% of the Earth's surface is water but all the seismic stations are on land, because it is too difficult and expensive to install permanent sensors on the seafloor" Dr Giuseppe Marra of the NPL told the BBC.
But numerous optical-fibre cables carry data across the world's seas and oceans.
It is estimated there are more than 430 around the world, spanning distances of 1.3 million km (800,000 miles)