Just like something from a horror film - five men trapped inside a tiny 22ft tube with a single window with no seats, thousands of feet below sea level with no way of reaching the outside world.
Those onboard, including British billionaire and explorer Hamish Harding, will have to contend with the "terrifying," "cramped," and "hostile" conditions - all while not being able to see beyond the pitch-dark depths of the ocean.
They don't just have to cope with the sense of claustrophobia they will be feeling, but also a dwindling supply of oxygen, with about 20 hours left before the tank runs empty.
But Ex-Royal Navy Clearance Diver and Falklands Veteran Ray Sinclair believes that they could have already succumbed to the toxic carbon dioxide levels and plummeting temperatures.
Sinclair took part in a deep-sea dive in the North Sea in 1984 in a small submarine that detaches from the main ship, just like the missing Titan.
He said, "They will be extremely cramped in there, they won't be able to stand up and the conditions they will be experiencing will be terrifying," speaking from experience.
His main concern, he exclusively told Daily Express US, is that the Titan vessel will have a carbon dioxide scrubber on board to remove excess toxic gas that builds up when passengers exhale in the confined space, but as in most crafts, this will have a limited capacity.
Sinclair said: "With just 20 hours of oxygen left roughly, the clock is seriously ticking but this may not be their biggest problem.
"These submarines have batteries which have a finite life, and they have CO2 scrubbers. If these die, the people could suffocate before the oxygen even runs out as the toxic gas fills their lungs.
"If the CO2 scrubbers died, it would be a matter of hours before they did too. The build-up of the gas would make them feel very sleepy very quickly and they would pass out from that. The CO2 would have nowhere to go. I'm worried this may have already happened.
"It's an absolute nightmare scenario, the worse any diver could ever imagine but realistically, I think it could still be a few days before they are found.
"They are a tiny vessel in a huge ocean and even if it's bobbing around on the surface, someone still needs to go down there to open it which would take time and it can't be done from the inside.
There is also a risk of hypothermia, Sinclair warned, due to the low temperatures in the depths of the ocean, as well as hyperventilation induced by panic attacks, which can use up more valuable oxygen.
He added: "I have been speaking with a few of my diving friends, who are very experienced, and we all agreed that we wouldn't have even gone down to these depths in this submarine.
"I worry that they weren't truly prepared for a situation like this. If they went down in civilian clothes, they could well get hypothermia and freeze to death. The temperatures down there are sub-zero and could get them quickly."
It's very much a race against time to rescue the passengers if they are not already dead, Sinclair said.
The Titan vessel, which was visiting the Titanic wreckage, submerged at 8 am EST on Sunday around 400 miles southeast of St John's, Newfoundland. It lost contact with its mothership at 9.45am - an hour and 45 minutes into the dive.
With no sign of the craft yet, the five Titanic tourists - including Harding, a French maritime expert, a British businessman and one of Pakistan's wealthiest men and his son - face the double threat of dwindling oxygen and extreme cold at two miles below the surface.
Sinclair continued: "My instinct, and it is terribly sad, is that they are already dead, wherever they are. I don't have much hope, unfortunately.
"It's dark, it's cold, they would be frightened. Having been in that situation, in the dark and cold water, it's an extremely hostile environment that deep in the ocean."
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