If five people onboard the missing submersible are still alive they may have little more than 24 hours of oxygen to breathe inside the small six-metre Titan.
"The crew and passengers will have to be calm and reducing their oxygen usage to extend their life," Frank Owen, a former Royal Navy commander, told Today.
Owen said it was "increasingly difficult" to envision a successful rescue given the extreme depth and challenging environment the Titan has gone missing in.
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The deepest-ever underwater rescue was the Pisces III submersible from depths of 480 metres in 1973.
Two people were lifted to the surface after being trapped for 76 hours.
But the Titanic wreck is immeasurably deeper, sitting nearly 4km below sea level.
"We always say you should have hope," Owen said, describing Titan pilot Stockton Rush as "very experienced", but he admitted "it's looking increasingly difficult".
"Most of the safety is self-help (and) the number of safety features in the craft are going to be fairly limited," he said.
"The distance from land makes the logistics challenging."
A heavy remotely operated vehicle will need to find and assist the Titan if it has sunk to the ocean floor or somehow become stuck in the wreckage, he said.
But if the Titan is beyond the continental shelf, at depths more than 200m, which is almost certain, very few vessels can go that deep, and certainly not divers.
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Other factors complicating the search include weather conditions and unknowns like the state of the Titan and whether it has working equipment like acoustic pingers that can be detected by search teams.
The Titan began its two-hour descent to the Titanic wreckage on Sunday morning but lost contact with its support ship Polar Prince, around 100 minutes into its deep dive.
Officials yesterday estimated the Titan had between 70 to 96 hours of oxygen.
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The Titan may have suffered a catastrophic leak, electrical fire, power loss or even been swept up in a tremendously powerful underwater current, a shipbuilding engineer said when assessing what could have happened to the vessel.
Associate Professor Eric Fusil, the director of the University of Adelaide's shipbuilding hub, described going undersea as perhaps even more challenging than venturing into space from an engineering perspective.
Four terrifying scenarios
Fusil detailed four terrifying scenarios that could have caused the Titan to lose contact and none appeared straightforward to solve.
Any submariner knows how unforgiving the abyssal domain is, Fusil said, referring to the depth of water known as the abyssal, or the abyss, ranging from 3000 to 6500 metres.
Marine engineering professor at University College London Alistair Greig said the outlook appeared bleak if the Titan has sunk to the seabed and can't get back up under its own power.
"The vehicles designed for navy submarine rescue certainly can't get down to anywhere near the depth of the Titanic, and even if they could, I very much doubt that they could attach to the hatch of the tourist submersible," he said.
Aquatic search is notoriously difficult.
Forensic geosciences expert Jamie Pringle said finding any object lost that deep is extremely "tricky" as the ocean floor is a lot more rugged than on land.
"Water isn't homogenous either – there are different stratified levels (and) currents."
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Timeline: How the expedition unfolded
Friday, June 16
- Polar Prince departs St John's, Newfoundland, with submersible Titan on board
Sunday, June 18
- Polar Prince reaches the submersible launch site
- One hour and 45 minutes later communications with Titan are lost
- Vessel reported overdue at 9.13pm local time (12.13pm Monday AEST)
- Rescue mission is launched involving US and Canadian coast guards
Monday, June 19
- Rear Admiral John Mauger of the US Coast Guard says the submersible has between 70 and 96 hours of oxygen left
- It's confirmed the submersible had the full complement of five people on board, including UK billionaire Hamish Harding
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