(Reuters) -A five-person submersible vessel taking tourists on a $250,000 trip to see the wreckage of the 1912 Titanic disaster 12,500 feet (3,800m) undersea is missing in the Atlantic off Canada.

Following is what we know so far:

WHO IS ON BOARD?

* HAMISH HARDING. The British billionaire and chairman of aviation company Action Aviation is among those missing, according to his stepson. Dubai-based Harding had posted on social media that he was proud to be heading to the Titanic as a "mission specialist", adding: "Due to the worst winter in Newfoundland in 40 years, this mission is likely to be the first and only manned mission to the Titanic in 2023."

Harding was also on board the 2019 "One More Orbit" flight mission that set a record for the fastest circumnavigation of earth by aircraft over both geographic poles.

* SHAHZADA DAWOOD and his son SULEMAN. Their family have confirmed they are on board. Shahzada is vice chairman of one of Pakistan's largest conglomerates, Engro Corporation, with investments in fertilisers, vehicle manufacturing, energy and digital technologies. According to the website of SETI, a California-based research institute of which he is a trustee, he lives in Britain with his wife and two children.

* PAUL-HENRI NARGEOLET. The 77-year-old French explorer, whom media say is one of the five on board, is director of underwater research at a company that owns the rights to the Titanic wreck. A former commander in the French Navy, he was both a deep diver and a mine sweeper. After retiring from the navy, he led the first recovery expedition to the Titanic in 1987 and is a leading authority on the wreck site. In a 2020 interview with France Bleu radio, he spoke of the dangers of deep diving, saying: "I am not afraid to die, I think it will happen one day."

* STOCKTON RUSH. The founder and CEO of the vessel's U.S.-based operating company OceanGate is also on the submersible, according to media reports. "It is an amazingly beautiful wreck," Rush told Britain's Sky news of the Titanic earlier this year. "Rush became the youngest jet transport rated pilot in the world when he obtained his DC-8 Type/Captain’s rating at the United Airlines Jet Training Institute in 1981 at the age of 19," according to his biography on OceanGate's website.

WHAT'S THE FIRM BEHIND THIS?

* Based in Everett in Washington State in the United States, OceanGate says it uses next generation crewed submersibles and launch platforms to increase deep ocean access as far as 4,000 metres.

* "OceanGate has successfully completed over 14 expeditions and over 200 dives in the Pacific, Atlantic and Gulf of Mexico," its website says. "Following every mission, the team evaluates and updates the procedures as part as a continued commitment to evolve and ensure operational safety."

WHAT IS THE VESSEL?

* Although popularly called a submarine, in marine terminology the "Titan" vessel carrying the five is a submersible. While a submarine can launch itself from a port independently, a submersible goes down off a support ship.

(Reporting by Ariba Shahid in Karachi, Kate Holton in London, Dominique Vidalon in Paris;Writing by Andrew Cawthorne;Editing by Christina Fincher)