A British explorer is among five people on a missing submersible used to take tourists to view the wreck of the Titanic.
The US coastguard said there was a “five person crew” on the 21-foot craft that lost contact with its base off the coast of Canada on Sunday.
They have between 70 to 96 hours of air, officials overseeing the search attempt believe.
The vessel was reported overdue on Sunday evening about 435 miles south of St John’s, Newfoundland, sparking a race against time to rescue those on the craft.
British explorer Hamish Harding, the chairman of private plane firm Action Aviation, is one of those onboard the OceanGate Expeditions vessel, his firm said.
Pakistani businessman Shahzada Dawood, a trustee at the SETI Institute, and his son Sulaiman are also on the craft.
“We are very grateful for the concern being shown by our colleagues and friends and would like to request everyone to pray for their safety," their family said in a statement.
Widespread media reports suggest that Stockton Rush, the CEO of OceanGate, the company behind the expedition, is also among those on the submersible.
The fifth member is 73-year-old French explorer Paul-Henry Nargeolet, according to a Facebook post by Mr Harding ahead of the dive.
The US Coast Guard, which is leading search and rescue efforts, said it was unclear whether the submersible was still underwater or had surfaced and was unable to communicate.
US and Canadian ships and planes have swarmed the area about 900 miles east of Cape Cod.
Some have dropped sonar buoys that can monitor to a depth of 13,000 feet, Rear Admiral John Mauger told reporters on Monday.
“It is a remote area and a challenge, but we are deploying all available assets to make sure we can locate the craft and rescue the people onboard," he told reporters.
Mark Butler, managing director of Action Aviation, said: "There is still plenty of time to facilitate a rescue mission, there is equipment on board for survival in this event. We’re all hoping and praying he comes back safe and sound."
Mr Harding holds three Guinness World Records, including the longest duration at full ocean depth by a crewed vessel, longest distance traversed at full ocean depth by a crewed vessel and fastest circumnavigation via both Poles by aeroplane.
Mr Harding’s cousin, Kathleen Cosnett, told The Daily Telegraph she saw Mr Harding as “daring" and “inquisitive", and that she was “devastated" to learn he was missing.
On social media at the weekend, he said he was “proud to finally announce" he would be aboard the mission to the wreck of the Titanic.
OceanGate Expeditions said its focus was on those aboard the vessel and their families.
“We are deeply thankful for the extensive assistance we have received from several government agencies and deep sea companies in our efforts to re-establish contact with the submersible," the company said in a statement.
“We are working toward the safe return of the crew members."
Marine experts said on Monday that the rescuers faced steep challenges.
Alistair Greig, a professor of marine engineering at University College London, said 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," he said.
The company charges guests $250,000 (£195,270) for its eight-day expedition to see the wreck, which sits 3,800m (12,500ft) down at the bottom of the Atlantic about 600km (370 miles) off the coast of Newfoundland, Canada.
The vessel can seat five people which usually includes a pilot, three paying guests, and what the company calls an “content expert”. It is controlled by a Play Station controller and has one window for crew to look through.
A full dive from the surface to the wreck reportedly takes eight hours in total.
The Titanic, which was the largest ship of its time, hit an iceberg on its maiden voyage from Southampton to New York in 1912. Of the 2,200 passengers and crew onboard, more than 1,500 died.
Its tragic fate continues to have a grip on the popular imagination inspiring plays, films and books down the years.
