ITV News Correspondent John Ray reports on the enormous difficulties facing the rescue operation
Time is running out for US and Canadian authorities searching for a missing submersible carrying tourists to the Titanic shipwreck.
The vessel, named Titan and operated by OceanGate, lost contact about two hours into its dive on Sunday. Teams are frantically searching the ocean 900 miles east of Cape Cod.
Rescuers said they estimated there was 40-hours of "breathable air" remaining on the vessel, with the occupants at increasing risk of hypothermia or suffocation.The sub's intricacies have been described as surprisingly "improvised" by a journalist who took the trip last year, and the entire craft is designed to be operated with just an Xbox controller.
But the company have previously detailed its "unparalleled safety feature," and has trained experts on board who have decades of deep-sea diving experience, including Titanic's "greatest explorer" and OceanGate CEO.
What could have happened to the sub and how can it - and its crew - be rescued?
What might have caused Titan to lose contact?
Alistair Greig, a professor of marine engineering at University College London, suggested there may be several reasons, including a power failure or a leak.
He said that submersibles typically have a drop weight, which is "a mass they can release in the case of an emergency to bring them up to the surface using buoyancy".
"If there was a power failure and/or communication failure, this might have happened, and the submersible would then be bobbing about on the surface waiting to be found," Professor Greig said.
If this was the case, other experts have suggested the crew would already have made contact on the radio.
Another scenario is a leak in the pressure hull, he posed, in which case the prognosis is not good.
ITV News' Lucy Watson takes us through the geography of where the submersible is expected to be
"If it has gone down to the seabed and can't get back up under its own power, options are very limited.
"While the submersible might still be intact, if it is beyond the continental shelf, there are very few vessels that can get that deep, and certainly not divers."
Even if they could go that deep, Professor Greig doubted they could attach to the hatch of OceanGate's submersible.
Associate Professor Eric Fusil is Director of the Shipbuilding Hub at the University of Adelaide said there were four risk factors: a power blackout, flooding, fire and entangling.
"A wreck, such as the Titanic, can present tricky spots if coming too close, where the Titan could get trapped and struggle to free herself," he said.
What does the search and rescue mission look like?
Professor Fusil said the priority was to "localise and communicate prior to rescue".
"It would require a submarine with both the ability to dive as deep as the Titan and, or, some active sonar to sweep the area," he said.
Even military nuclear-powered submarines are limited to depths between 0-500 metres and could only use their sonars to detect any sign of the Titan, without a possibility to get closer, Professor Fusil said.
If the operation is triumphant, it would be 11,000ft deeper than the deepest successful undersea rescue in history, when British engineers Roger Mallinson and Roger Chapman survived after their submersible, called Pisces III, was trapped on the seabed at a depth of 1,575ft off the southwest coast of Ireland in 1973.
David Concannon, an adviser to OceanGate who had planned to be on the expedition, said officials are working to get a remotely-operated vehicle (ROV) that can reach a depth of 20,000ft to the site as soon as possible.
ROVs are fitted with aquatic thrusters, cameras, lights and may also have mechanical manipulators, sonar and magnetometers.
A plane will also drop sonar equipment into the sea in an effort to detect sound on the seabed.
Dr Simon Boxall, an oceanologist at the University of Southampton, said rescuers would have to first locate the submarine which is not a “trivial ask” as it could be as much as 4km deep, while the boat is beyond the reach of helicopters.
Very few vessels can reach the sorts of depths that are likely to be needed to carry out a successful rescue mission, he added.
He said the worst scenario would be if the hull was crushed.
Footage of the vessel underwater
What have past explorers said?
CBS News reporter David Pogue took the trip last year, alongside CEO Stockton Rush, and told the BBC that passengers were sealed inside the capsule by 17 bolts put on by an external crew.
He said the craft had seven different functions that would help it resurface, making it "really concerning" that none have so far worked.
However, Mr Pogue said the vessel's resurfacing capabilities would be irrelevant if the sub became trapped or sprang a leak.
"There's no backup, there's no escape pod," he said.
"It's get to the surface or die."
Mike Reiss, a TV comedy writer who worked on The Simpsons and also took the trip last year, said he was not optimistic.
David Pogue from CBS speaks to OceanGate CEO Stockton Rush before their trip last year
Mr Reiss told BBC Breakfast: "I'm not optimistic just because I know the logistics of it. And I know really again, how vast the ocean is, and how very tiny the craft is.
"So the idea is, if it's down at the bottom, I don't know how anyone's going to be able to access it, much less bring it back up.
"There is a hope that it's at, or near, the surface.
"I did three separate dives. I did one dive to the Titanic and two more off the coast of New York.
"Every time they lost communication and again, this is not a shoddy ship or anything."
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