Eighty years after they were deployed for the first time to detect Nazi submarines during the Second World War, sonobuoys are aiding the hunt for missing Titanic explorers.

The underwater sonar devices picked up banging noises in the area where the Titan submersible vanished, as searches continue for the vessel that dropped off the radar near the shipwreck on Sunday.

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Marine operations expert Mike Welham told Sky News that the sounds, which were heard at 30-minute intervals, suggest someone has “taken charge” of the sub and is “sending out the taps” in the hope they’re detected – perhaps knowing how sonobuoys have evolved into a key tool for hunting people missing at sea.

What even are sonobuoys?

Put simply, they are designed to locate objects underwater using sonar.

The devices come in two parts, which are packed into a cylindrical canister until they hit the water.

Once dropped from planes in a formation, they take only a few minutes to deploy. The video below shows just how quickly they can be up and running.

Once deployed, an inflatable with a radio transmitter sits on the surface, while the equipment for detecting noise is strung along a wire and descends below the surface.

Any signals picked up are relayed from the transmitter on the surface to the aircraft.

They are a crucial part of a naval search team’s arsenal, as radio and GPS signals cannot travel through water.

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How do sonobuoys pick up sound?

Sonobuoys use two forms of detection to identify noises underwater.

Passive detection is when they listen out for sounds produced by vessels, such as propellers and machinery.

These are a cheaper, simpler form of sonobuoy, using an underwater microphone called a hydrophone.

Active detection instead sends out a sonar “ping”, which would bounce off the surface of a sub like the Titan, and a returning echo is then used to map the water and locate any objects.

Rather than a hydrophone, they use a transducer, which functions like an antenna.

How have they been deployed before?

Sonobuoys were designed to be strictly military hardware.

They were first used during the Second World War to detect German U-boats.

They got plenty of use during the Cold War, too, to track nuclear submarines around the world.

The devices are still employed by national armed forces, from the US Navy to the UK’s Ministry of Defence, but their use cases have diversified to encompass search and rescue operations.

They were previously used during the search for the black box of missing Malaysia Airlines flight 370 after it disappeared over the Indian Ocean, and in the search for an Argentinian submarine that vanished in 2017.

Read more:
Why hunt for missing sub is so difficult
What we know so for about missing submersible

How are they being used in the search for Titan?

They were dropped into the water off Newfoundland, Canada, by a P-3 plane.

From there, they descended to the required depth – which can be up to around 1,500ft. For context, the Titanic shipwreck lies about 12,000ft below the surface.

While officials said an investigation of the banging sounds they had picked up had come back negative, experts insist sonobuoys could still prove crucial in narrowing the search.

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“The seabed is enormous, the water depth is enormous,” said Mr Welham, who compared searching for the submersible to trying to find a coin on a football pitch.

Noise from the Titan should be distinguishable from the ambience of the North Atlantic, and the decision to investigate the banging sounds suggests officials thought there was something to them.

Dr Jamie Pringle, a reader in forensic geosciences, said they were “unlikely” to be from a surface ship propeller or another submarine.

“Acoustic noise travels far in water, so that is both good and bad news,” he said.

“You would still need three of those static buoys to be able to triangulate the sound source to get a position fix.”

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