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The search for five explorers missing on a deep-sea journey to the wreck of the Titanic was widened on Tuesday as friends said they were praying for a miracle.
The submersible craft Titan, which set off with 96 hours of oxygen but has not been seen since Sunday, remained elusive despite an intensive search in the the North Atlantic.
The US Coast Guard said it had expanded its search into deeper waters as the clock ticks down.
Pakistani businessman Shahzada Dawood and his son Suleman were confirmed to be two of the passengers on board. A third is Hamish Harding, a British billionaire who lives in the UAE and flew into space last year.
A French navy pilot, Paul-Henry Nargeolet, and the boss of tour company OceanGate, Stockton Rush, were believed to be the other members of the voyage.
Experts said a rescue in the ocean depths would be challenging even if the craft were located.
David Mearns, a specialist in searching for shipwrecks who is friends with Mr Harding and knows Mr Nargeolet professionally, told The National the craft's disappearance was a "nightmare" for everyone.
"It's a horrible situation that we are all in, and none more important than his family", said Mr Mearns, a marine scientist and oceanographer.
He said there was "more hope" on Tuesday because of a new ship arriving at the scene that may have the equipment to carry out a rescue.
"If they’re still alive that is a possibility and we are praying for a miracle. That could be the miracle," he said.
"But I still know how difficult it is. They have to get down very deep and find the submersible. They have got to recover it. So that’s a lot to hope for.
"I think now our minds should be on trying to make the recovery as opposed to being about what happened. That’s where my thoughts were yesterday and until time runs out or they are recovered."
US and Canadian coast guards were combing the North Atlantic surface and using sonar equipment to trace the vessel’s movements. The pressure at the seabed was compared to two elephants on a thumbnail.
France announced it was sending a deep-sea diving vessel called the Atalante that belongs to a government research institute.
It was not clear what had happened to the craft to cause it to lose communications.
Mr Harding said in his last social media post that a “weather window” had opened up for a dive on Sunday after severe conditions in Newfoundland, Canada.
Private plane company Action Aviation said the family of its missing chairman Mr Harding was grateful for "kind messages" since his disappearance.
The president of the US-based Explorers Club, Richard Garriott de Cayeux, said Mr Harding had been looking forward to the expedition when they met shortly beforehand.
"His excitement about this expedition was palpable," he said. "We all join in the fervent hope that the submersible is located as quickly as possible and the crew is safe."
The Titanic wreck lies on the seabed about 1,400km east of Cape Cod, Massachusetts, at a depth of about 4km. Since the wreck's discovery in 1985, it has been slowly succumbing to bacteria and some artefacts have been removed by human explorers.
OceanGate offers tours costing $250,000 to see the remains of the Titanic, which sank in April 1912 after hitting an iceberg on its maiden voyage.
“It is a challenge to conduct a search in that remote area,” said Rear Adm John Mauger of the US Coast Guard.
With no trace of the Titan, pilots in C-130 military planes were looking over the water to see if the craft had surfaced while Canadian forces put sonar buoys in the ocean to pick up any sounds.
Canada’s Rescue Co-ordination Centre Halifax dispatched a P8 Poseidon aircraft that is designed for anti-submarine warfare.
“We’re making the best use of every moment of that time to locate the vessel,” Rear Adm Mauger added.
British Prime Minister Rishi Sunak's official spokesman said the UK was in contact with the family of Mr Harding as the rescue operation continues.
The UK "stands ready to provide any additional assistance, including as our capacity as the host nation for Nato's multinational submarine rescue capacity," the spokesman said.
Chris Parry, a former rear admiral in Britain’s Royal Navy, said the small vehicle could be difficult to find if it was deep in an ocean canyon among the debris of the Titanic itself.
He said that even if the passengers were found alive, it would be impossible to transfer them to another vehicle at that depth.
Rescuers would have to try to lift the Titan out with a crane and a long wire or send another submersible with “grabbing arms” to bring it to the surface, he told Sky News.
OceanGate said that it was looking into “all options” to retrieve the crew.
Missing Titanic submarine 'could have had catastrophic failure'
The six-metre vessel lost contact with the Polar Prince, a Canadian research vessel that had towed it near the wreck, about one hour and 45 minutes after the dive began.
The military is hoping to enlist private vessels to assist in the search, as well as for any possible rescue mission if the craft is located.
Robert Blasiak, an oceans expert at Stockholm University, Sweden, said the dive to the wreck was within the craft’s specifications but that most deep-sea science exploration was done by robots.
He said the craft would have thick walls to withstand the immense pressure close to the seabed, which he said was 380 times what a person would experience above ground.
Mr Rush, OceanGate's founder, said on a BBC programme last year that the craft was operated with what was "basically a Sony PlayStation-style controller".
"I want to tell people this is not your grandfather’s submarine. It has got one button, and that’s it," he added.
The Dawood family runs one of Pakistan’s biggest business conglomerates, with investments in agriculture, industries and the health sector. Mr Dawood also is on the board of trustees for the California-based SETI Institute that searches for extraterrestrial intelligence.
“We are very grateful for the concern being shown by our colleagues and friends," the Dawood family said, confirming the father and son's involvement in the dive.
Mr Dawood is an adviser of Prince's Trust International. On Tuesday, Will Straw, the chief executive of the charity, expressed shock over the news.
Mr Straw said: "We are shocked by this awful news, praying for a rescue and sending our thoughts to his family during this deeply challenging time".
He said Mr Dawood has been an adviser to the charity's international arm in "various capacities" with a focus on its work in Pakistan. His father Hussain Dawood was a founding patron of Prince's Trust International.
Mr Harding holds exploration records including the longest time spent traversing the deepest part of the ocean – the Mariana Trench – on a single dive, and the fastest circumnavigation of the Earth via its poles by plane.
Last June, he went into space on Blue Origin’s New Shepard rocket. He posted online before the Titanic mission that it was likely to be the only journey to the wreck this year because of severe weather.
Action Aviation said it was thankful for the "continued efforts" of rescuers and put "great faith and trust in their expertise".
It described Mr Harding as a "living legend of aviation" and an "extraordinarily accomplished individual who has successfully undertaken challenging expeditions" including the space mission.
"He has travelled on a submersible to the Challenger Deep in the Mariana Trench and has been to the South Pole a number of times," it said.
"The team at Action Aviation are extremely proud of Hamish and we look forward to welcoming him home."
