On March 18 2014, Malaysian Airlines Flight MH370 disappeared over the southern Indian Ocean. Remarkably, its fate is still unknown despite years of coordinated searches of many miles of sea.
Thanks to some new technological developments, perhaps the next plane to disappear won't go unfound for so long.
A team at Cardiff University's School of Engineering has developed a way to use underwater microphones to precisely identify the time and location an object hits the surface of large bodies of water.
The method relies on sound waves that can spread through water at the speed of sound. These waves occur naturally whenever something hits the sea surface, and some organisms like plankton even depend on them to propel themselves around the ocean.
These signals can be detected by specially-designed underwater microphones called hydrophones, and by triangulating the signals received at three or more different places, it's possible to find the exact time and position the signal originated from.
In testing, the team first dropped 18 balls into a water tank at various distances and heights and measured the acoustic waves that were generated.
They then analysed data collected by hydrophones off the coast of Western Australia - these recorders are operated by the Comprehensive Nuclear Test Ban Treaty Organisation, which uses them to detect underwater nuclear tests. Using the data, they were able to identify the time and location of a selection of recent earthquakes.
"By using existing detectors dotted all around our oceans and listening out for signatures from these deep ocean sound waves, we've uncovered a completely novel way of locating objects impacting on the sea surface," said Usama Kadri from Cardiff University's School of Mathematics, the lead author of a paper describing the method published in Scientific Reports.
"Tracking these acoustic gravity waves opens up a huge range of possibilities, from locating falling meteorites to detecting landslides, snowslides, storm surges, tsunamis and rogue waves."
The team then went back and analysed the data from March 18 2014 - the night that MH370 disappeared. They detected two "remarkably weak signals" between midnight at 02:00 UTC. The weakness of the signals means there's a relatively large area of uncertainty.
"Though we've located two points around the time of MH370's disappearance from an unknown source, we cannot say with any real certainty that these have any association with the aircraft," said co-author of the study Davide Crivelli, from Cardiff University's School of Engineering.
"What we do know is that the hydrophones picked up remarkably weak signals at these locations and that the signals, according to our calculations, accounted for some sort of impact in the Indian Ocean. All of this information has been passed onto the Australian Transport Safety Bureau and we anticipate that both now, and in the future, this new source of information could be used in conjunction with a whole of host of other data that is at the disposal of the authorities."