The five passengers who were onboard the OceanGate submersible Titan are now known to have died after the vessel suffered a ‘catastrophic implosion’ on its way to the wreck of the Titanic.

British billionaire explorer Hamish Harding, French diver Paul-Henri Nargeolet, Pakistani businessman Shahzada Dawood, his 19-year-old son Suleman, and OceanGate CEO Stockton Rush were onboard, with the days-long rescue mission coming to a tragic end after debris from the craft was found.

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A US coastguard spokesperson confirmed the news, saying the debris was ‘consistent with the catastrophic loss of the pressure chamber’.

Previous trips in the Titan had raised concerns about the safety of the vessel, although OceanGate stated that the Titan completed over 50 test dives, including to depths similar to those of the Titanic, both in waters around the Bahamas as well as in a pressure chamber.

Of course no underwater expedition is without risks – but just how common is it for a sub to implode?

How common is a sub implosion?

While a sub implosion is a catastrophic event, which is unlikely to leave any survivors, it doesn’t happen that often.

While there have been over 30 major accidents and incidents involving submersibles and submarines since 2000, the Titan disaster marks only the secondknown incident of a vessel suffering an implosion.

The previous tragedy came in 2017, when the Argentine navy sub ARA San Juan vanished without trace in the South Atlantic.

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Authorities said a year later they believed it to have imploded two hours after its last communication, with images of the craft captured on video by an underwater robot.

On that occasion the submarine was located at a depth of more than 3,000 feet in a desolate area of undersea craters and canyons 250 miles off the coast of Argentina.

All 44 sailors on board were killed in the disaster.

The possibility that Titan may have imploded was raised by experts before the debris was discovered.

What happens during an implosion?

An implosion is defined as an ‘instance of something collapsing violently inwards’, usually due to being placed under extreme pressure.

The Titan would have been facing extreme water pressure at the depth it was at, which likely contributed to the disaster – although others have been sharing theories about what might have led to it happening.

Ryan Ramsey, a former Royal Navy submarine captain, said he believes either the bolts to the hatch failed, or the pressure hull was fractured.

He said: ‘I think either one of two things has happened.

‘Either the hatch with the 17 bolts they used to seal them in has had a failure, which has then caused the hull to collapse at pressure because there’s huge amounts of pressure, even halfway down.

‘Or the pressure hull itself had a defect in it when they sailed and that’s fractured from the pressure, and caused the same result.

‘The only positive out of it is that it was instantaneous and they didn’t know anything.’

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Australian robotics professor Stefan B Williams was among those who had suggested that this could have been the cause of the sub’s disappearance.

Writing in The Conversation, he said: ‘Although the Titan’s composite hull is built to withstand intense deep-sea pressures, any defect in its shape or build could compromise its integrity – in which case there’s a risk of implosion.’

The fact that communication was lost very quickly suggests something may have gone very wrong in a short space of time. ‘If the pressure vessel has failed catastrophically, it’s like a small bomb going off,’ Mr Williams told The Guardian.

‘The potential is that all the safety devices might be destroyed in the process.’

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