A deep sea specialist who raised safety concerns about the Titan submersible says its fate was inevitable.

All five passengers, including OceanGate CEO Stockton Rush, died when the sub imploded on a commercial trip to the Titanic shipwreck.

Kiwi deep sea specialist Rob McCallum had several concerns about the Titan submersible, including the use of carbon fibre and off-the-shelf products to construct the vessel.

"There's about three dozen members of the industry, everyone from manufacturers to operators, who shared the same concern. They wrote an open letter to the CEO of OceanGate."

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