If still alive, the crew of the missing Titanic sub will be facing a 'ticking clock' against the cold and oxygen supplies, an expert has warned.
The former submarine commander explained the perils the five missing explorers will be contending with if the sub is still intact.
The vessel went missing an hour and 45 minutes into the trip down to explore the wreck of the Titanic around 435 miles (700km) south of St. John’s, Newfoundland, and 2.4miles below the surface.
At those depths pressure is around 400 times what it is at sea level.
Captain David Marquet told the Mirror: "The clock is ticking. The clock is ticking. The clock is ticking.
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Captain Marquet served in the US submarine force for 28 years and went on to write a bestselling book about his experience titled Turn the Ship Around!
Speaking about the missing submarine, he said: “There are a lot of things that can go wrong and they’re a long way from civilisation.”
He explained what would happen as the oxygen levels run out.
"They might be in darkness or have a very dim light but that won’t kill them. Running out of oxygen and then inhaling carbon dioxide is the problem.
"On big submarines we have machines that will remove the CO2 from the air but it’s unlikely a submarine that size has one. It may have some kind of absorption module but that would become saturated.
"So what happens is the CO2 builds up, we get headaches, we get nausea, we get confused and eventually you can die from CO2 poisoning."
If there was a leak in the vessel, crew could be dealing with another terrifying problem.
Captain Marquet said: "That submarine is designed to go very very deep and the pressures are incredibly high. The Titanic is down almost two miles and so the pressure there is around 400 times what we experience here.
"That’s like taking a whole room full of air and compressing it to the size of a trash can. The human body can’t survive with those pressures.
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Speaking at a press conference, Captain Jamie Frederick of the US Coast Guard First District said that the original 96 hours of oxygen supply was an estimate from data they have on the OceanGate Expeditions submersible.
"From the data we were using, the starting point was 96 hours, we're now at approximately 40 to 41 hours left."
This means that the five crew on board have enough oxygen to last them until 11am BST on Thursday, June 22.