Five passengers are known to be on the submersible.
The submersible vessel that vanished during a dive to the Titanic shipwreck may have experienced one of the so-called trinity of disasters that all such expeditions seek to avoid: a hull breach, a fire or an entanglement.
The first of those outcomes aren't survivable, said Joe MacInnis, the renowned Canadian explorer and physician who has been to the Titanic site four times and is a close friend of one of the passengers aboard the missing Titan.
"A hull failure is catastrophic," MacInnis, 86, said in an interview Tuesday. "There is this kind of implosion, and it's terrible."
However, the second two disasters can be managed. Crew train for fire emergencies and although challenging, all good subs have firefighting capabilities. And MacInnis has himself experienced what it's like to be trapped by the doomed passenger ship - and to break free - more than 30 years ago.
"It was my second dive to the Titanic," he said. He was inside a Russian Mir submersible when it landed on the floor of the Titanic's pilot house and became entangled in wires. "When it came time to go up, we couldn't. It was a thick adrenaline moment."
Fortunately, the expedition was able to send a second sub down to assess the situation and help talk the chief pilot through the required navigation to wiggle free.
That self-rescue ability - either with a second sub or a remotely operated vehicle that can be flown quickly to the site - is crucial on these types of expeditions, said MacInnis. France has dispatched a research vessel, the Atalante, equipped with an underwater robot to reach the missing craft.
Five passengers are known to be on the submersible: adventurer Hamish Harding, founder of investment group Action Aviation; Stockton Rush, founder of OceanGate Expeditions, which is running the expedition; Engro Corp's vice chairman Shahzada Dawood and his son Suleman; and French maritime expert Paul-Henry Nargeolet.
Three of that crew are likely newcomers to deep-sea diving, he said. "They would have been on adrenaline before the dive. And in a high-stress moment they will be red-lined," MacInnis said. But one of them, Nargeolet, is a close personal friend of MacInnis.
"PH is the best person you could be in a sub with," he said. "He is very, very calm under extreme stress. If they're still alive, he will be a terrific calming influence on the others."
MacInnis, whose work on the 1992 IMAX film Titanica helped inspire friend James Cameron to create the Hollywood blockbuster, said if the submersible had merely lost radio contact, the standard protocol would be to immediately surface.
He says he was "stricken" when he learned the Titan was missing.
"Getting lost in the depths is a primal fear," he said. "My heart goes out to the guys in the subs 0 and the team on the surface trying to solve the problem."
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