Nearly five years ago, the doomed Malaysia Airlines Flight 370 vanished without a trace, with 239 people on board. The search in the Indian Ocean for the wreckage of the aircraft has been the largest and most expensive search effort in history — but it has turned up nothing.

Now, a team of researchers says Malaysia Airlines Flight 370 may have crashed thousands of miles from the search locations, based on sounds recorded in the ocean near the time the passenger jet disappeared on March 8, 2014.

In research published Jan. 29 in the open-access journal Scientific Reports, applied mathematician Usama Kadri said underwater microphones in the Indian Ocean had recorded four distinctive sound events, caused by very low-frequency acoustic-gravity waves, around the time that Flight 370 could have crashed into the sea.

His research showed that one of those sound events happened relatively close to the search area — but two others are thousands of miles away, in the northern part of the Indian Ocean, somewhere between Madagascar and the atoll of Diego Garcia in the Chagos Archipelago, Kadri told Live Science. [Flight 370: Photos of the Search for Missing Malaysian Plane]

Investigators suspect that the lost airliner crashed somewhere in the Indian Ocean, although its flight path after it disappeared from civilian and military radars, west of the Malay Peninsula, is not known.

The aircraft's captain, Zaharie Ahmad Shah, had ordered enough fuel for a routine flight from Kuala Lumpur, Malaysia, to Beijing, a flight that would have lasted 7 hours and 30 minutes. But just how long the Boeing 777 jet could have stayed airborne would depend on its actual flight path, its altitude and how many of its two engines were operating.

Ocean sounds

Kadri and colleagues at the University of Cardiff in the U.K. and Memorial University of Newfoundland in Canada analyzed sounds recorded by a network of underwater microphones (called hydrophones), which are maintained by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) to listen for banned nuclear tests.

The CTBTO hydrophones give directional bearings, loudness and frequencies of sounds in the ocean, from which scientists can calculate an approximate location for where those sounds originated.

But the CTBTO hydrophone network is designed to detect underwater nuclear explosions, in concert with other monitoring systems in the air and through seismological tremors in the earth — and it was thought not able to detect a crashing jet.

To learn more about the patterns of sounds made by objects crashing into the ocean surface, Kadri and his colleagues recorded the sounds caused by weighted spheres impacting tanks of water in 2017.

They found that when a massive object like an airliner crashes into the ocean, it creates a distinctive pattern of sound waves — including patterns of very low-frequency sounds known as acoustic-gravity waves (AGWs) that can be transmitted for thousands of miles through the ocean. [What's That Noise? 11 Strange and Mysterious Sounds on Earth and Beyond]

Kadri's latest research has found that the underwater speed of transmission of low-frequency AGWs, below 5 hertz, can be affected by the elasticity of the seafloor at particular locations.

That means each of the four distinctive sound events in the Indian Ocean identified by the researchers could have originated across a range of locations, but along a particular directional bearing.

Missing airliner

As well as two matching sound events recorded by the CTBTO hydrophones at Cape Leeuwin in Western Australia, the researchers found two sound events recorded by the hydrophones at Diego Garcia that could match the sounds of an airliner hitting the ocean.

Their directional bearings and timings indicated that they both occurred somewhere northwest of Madagascar — thousands of miles from the areas where searchers have looked for wreckage of the aircraft.

But the ocean is a noisy place, and Kadri said the underwater sounds might have also been caused by underwater earthquakes or volcanic eruptions, or even by meteorites or space junk falling in the ocean. [Top 10 Greatest Explosions Ever]

However, they were also valid sound signals that could have been created by the crash of Flight 370, he said.

Kadri said he recognized that the sound events near Madagascar were thousands of miles from the so-called "7th arc" — the line of possible positions of Flight 370 calculated from the aircraft's final radio signals to a tracking satellite shortly before it would have run out of fuel.

Searchers have relied on the 7th arc in their efforts to find wreckage of the missing airliner; it curves through the eastern Indian Ocean, south of the Indonesian island of Java and toward Antarctica, between 300 and 1,800 miles (500 to 3,000 km) away from the western Australian coast.

But Kadri said the positions suggested by the satellite radio data might be inaccurate, or calculated incorrectly, or otherwise misleading.

"I don't want to go into what could go wrong, but there are many things," Kadri said of the 7th-arc data. "It could be anything."

Search at sea

Kadri said that future searches for any wreckage of the airliner should start with scientific investigations of the sound events recorded in the Indian Ocean — without regard to information from other sources, such as the satellite radio data, which could create large inaccuracies.

"All the efforts that were done before, they all relied on the satellite data as given evidence … unfortunately, they found nothing," he said.

Details of the new research had been relayed to the Malaysian and Australian authorities responsible for locating the aircraft, but there are currently no plans to resume the search at sea, Kadri said.

Other experts on the search for the crash site of Flight 370 gave divided opinions about the new research.

David Griffin, an oceanographer at the Australian government's Commonwealth Scientific and Industrial Research Organisation (CSIRO), told Live Science that he could think of no reason why the 7th-arc satellite data should be disregarded.

Griffin also estimated that crash sites near Madagascar and Diego Garcia would result in floating debris along the East African coast within a few months — in other words, by mid-2014.

But no floating debris from the crash was found there until late 2015 and 2016, around 18 months later, he said.

However, oceanographer David Gallo, the director of special projects at the Woods Hole Oceanographic Institution in Massachusetts, said he was not convinced that the satellite data represented by the 7th arc gave an accurate indication of the final positions of Flight 370.

Gallo, who led the successful search for the crash location of Air France Flight 447 in 2011, said the Australian-led searches for Flight 370 had relied on the 7th-arc data because they needed to respond quickly.

But "I'm not now nor ever was a fan of the 7th arc," Gallo told Live Science in an email: "[The] plane could very well have crashed north of Madagascar."

Editor's Note: This story was updated to note that the Boeing 777 aircraft has two, not four, engines.

Originally published on Live Science.