British adventurer Hamish Harding is among those missing on a submersible that was diving to the wreck of Titanic, his family have said.
Mr Harding, 58, chairman of aircraft firm Action Aviation, is a renowned explorer who has flown to space and holds three Guinness World Records.
His stepson Brian Szasz said in a now-deleted post on Facebook: He “has gone missing on (the) submarine.”
Submersible operator OceanGate said its “entire focus” is on the crew’s return.
It added that it was “exploring and mobilizing all options to bring the crew back safely”.
Over the weekend, Mr Harding said on social media that a ship had set off from the city of St John’s, in Newfoundland, Canada, for the destination of the Titanic wreck.
From there, he and the crew were planning to start diving operations in the submersible down to the wreck at around 04:00 local time (08:00 GMT) on Sunday morning.
He wrote on Facebook that he was “proud to finally announce” that he would be aboard the mission to the wreck of the Titanic.
Due to the “worst winter in Newfoundland in 40 years,” he said “this mission is likely to be the first and only manned mission to the Titanic in 2023”.
He continued: “A weather window has just opened up and we are going to attempt a dive tomorrow.”
Action Aviation said on Sunday that the sub had had a successful launch and Mr Harding was “currently diving“.
Boston’s coastguard said an operation to find five people on the submersible was under way, after contact was lost on Sunday morning – about one hour and 45 minutes into its dive.
A highly-sophisticated P-8 Poseidon aircraft with underwater detection capabilities has been deployed, said Rear Adm John W Mauger from the US Coast Guard.
He added that teams were conducting their searches 900 miles (1450km) east of Cape Cod, and that the Canadian armed forces and commercial vessels were in the area.
“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 a press conference.
He said planes had been dropping sonar buoys to listen for underwater noise and scan the ocean in case the submersible had surfaced.
A spokesperson from the UK’s Foreign Office said it was “in contact with the family of a British man following reports of a missing submarine off the coast of North America.”
The eight-day trip to the wreck of the Titanic – run by OceanGate Expeditions – costs $250,000 (£195,600) per person and starts in St John’s.
Participants travel some 370 miles (595km) on a larger ship to the area above the wreck site, then do an eight-hour dive to the Titanic on a truck-sized submersible known as Titan.
The Titan is designed to carry five people and has “life support” for 96 hours for the crew, according to the firm’s website.
For Mr Harding, a private jet dealer, the trip to Titanic’s wreckage was the latest in a string of adventures that has seen him visit the South Pole multiple times, fly into space in 2022 on board Blue Origin’s fifth human-crewed flight, and set three world records – including the longest time spent at full ocean depth during a dive to the deepest part of the Mariana Trench.
The Titanic sits 3,800m (12,500ft) beneath the surface at the bottom of the Atlantic. It is about 600km (370 miles) off the coast of Newfoundland.
The passenger liner, 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.
