Rescuers are scouring the remote North Atlantic for a third day, racing against time to find a missing submersible after it disappeared while taking wealthy tourists to see the wreckage of the Titanic in deep waters off Canada's coast.
The six-metre Titan submersible has the capacity to stay underwater for 96 hours, according to its specifications - giving the five people aboard until early on Thursday before air runs out.
Captain Jamie Frederick of the US coast guard told reporters at a press conference on Tuesday there were about 40 hours of breathable air left aboard the submersible.
One pilot and four passengers were inside the miniature sub early on Sunday when it lost communication with a parent ship on the surface about an hour and 45 minutes into its two-hour dive.
As Canadian and US authorities stepped up the search, previous questions about the safety design and development of the submersible by its owner, US-based OceanGate Expeditions, came to light.
The Titanic site is about 1450km east of Cape Cod and 644km south of St John's, Newfoundland.
US and Canadian aircraft have searched almost 20,000 square kilometres, larger than the state of Connecticut, Frederick said.
He said the Canadian military has dropped sonar buoys to listen for any sounds that might come from the Titan, and a commercial vessel with a remote-controlled deepwater submersible was also searching near the site.
A French research ship carrying an autonomous deep-sea diving vessel has been dispatched at the request of the US Navy and was expected to arrive at the search area Wednesday night local time, the Ifremer research institute said.
Those aboard the submersible, the highlight of a tourist expedition that costs $US250,000 ($A370,000) per person, included British billionaire Hamish Harding, 58, and Pakistani-born businessman Shahzada Dawood, 48, with his 19-year-old son Suleman, both British citizens.
The 77-year-old French explorer Paul-Henri Nargeolet and Stockton Rush, founder and CEO of the vessel's US-based operating company OceanGate, were also reported to be on board.
Authorities have not confirmed the identity of any passenger.
Rescuers face significant obstacles both in finding the Titan and in saving the people aboard, according to experts.
If the submersible experienced an emergency in mid-dive, the pilot would likely have released weights to float back to the surface, according to Alistair Greig, a professor of marine engineering at University College London.
The submersible is sealed with bolts from the outside, which means the occupants cannot escape without assistance even if it surfaces.
If the Titan is on the ocean floor, a rescue effort would be even more challenging due to the extreme conditions below the surface.
The Titanic lies 3810 metres underwater, where light does not penetrate.
Only specialised equipment can reach those depths without getting crushed by the massive water pressure.
"It's really a bit like being an astronaut going into space," Titanic expert Tim Matlin said.
"I think if it's on the seabed, there are so few submarines that are capable of going that deep.
"I think it was going to be almost impossible to effect a sub-to-sub rescue."
The sub hull design's ability to withstand such depths was questioned in a 2018 lawsuit filed by OceanGate's former director of marine operations David Lochridge, who said he was fired after he raised safety concerns.
OceanGate said in its breach-of-contract suit against Lochridge, who is not an engineer, that he refused to accept the lead engineer's assurances and accused him of improperly sharing confidential information.
The two sides settled the case in November 2018.
The company did not respond to requests for comment from Reuters and its lawyer in the Lochridge case, Thomas Gilman, declined to comment.
A lawyer for Lochridge declined to comment except to say, "we pray for everyone's safe return".
OceanGate said it was "mobilising all options" and US Coast Guard Rear Admiral John Mauger told NBC News the company was helping to guide the search efforts.
"They know that site better than anybody else," Mauger said.
"We're working very closely with them to prioritise our underwater search efforts and get equipment there."
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