Rescue teams are searching for a British billionaire and four others who went missing on a deep-sea vessel near the wreck site of the Titanic ocean liner.

The US Coast Guard is leading the search for the small craft, named Titan, which was reported overdue about 435 miles south of St John’s in Newfoundland, Canada, on Sunday evening.

British billionaire Hamish Harding, chairman of private plane firm Action Aviation, has been named as one of the people on board.

The Titan’s dive on Sunday is one of many that have been made to the wreck, which is about 2.4 miles below the surface, by OceanGate Expeditions since 2021.

Here, the PA news agency sums up what we know so far.

– What do we know about Hamish Harding?

On social media at the weekend, the billionaire said he was “proud to finally announce” he would be aboard the mission to the wreck of the Titanic.

But he added that 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”.

He continued: “A weather window has just opened up and we are going to attempt a dive tomorrow.

“We started steaming from St John’s, Newfoundland, Canada yesterday and are planning to start dive operations around 4am tomorrow morning.

“Until then we have a lot of preparations and briefings to do.”

Mr Harding holds three Guinness World Records, for longest duration and distance traversed at full ocean depth by a crewed vessel, and fastest circumnavigation via both poles by aeroplane.

Two of these feats were achieved by Mr Harding and fellow explorer Victor Vescovo when they dived to the lowest depth of the Mariana Trench – the deepest part of the ocean – in a two-person submergence vehicle in March 2021.

In July 2019, Mr Harding was part of a team that achieved the fastest circumnavigation of Earth via both geographic poles by plane with a time of 46 hours, 40 minutes and 22 seconds.

In June last year, he travelled to space as a tourist with fellow billionaire Jeff Bezos’s Blue Origin company.

His 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.

– Who else was on board?

Shahzada Dawood and his son Sulaiman Dawood have been named as two of the other people on the submersible by their family.

A Dawood family statement obtained by CNN said: “As of now, contact has been lost with their submersible craft and there is limited information available.”

“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 while granting the family privacy at this time.

“The family is well looked after and are praying to Allah for the safe return of their family members.”

Mr Dawood is the vice chairman of Pakistani conglomerate Engro Corporation, which was founded as a fertiliser business, and, according to the company’s website, a board member of the King’s charity, Prince’s Trust International.

It is understood the Dawood family, who live in Surbiton, south-west London, are in Canada for a month.

French submersible pilot Paul-Henry Nargeolet and chief executive and founder of OceanGate Expeditions Stockton Rush are also on the vessel, according to reports.

– What do we know about the rescue?

In a press conference on Monday, Rear Admiral John W Mauger of the US Coast Guard said they are doing “everything” they can to find the submersible, saying it has one pilot and four mission operators aboard.

He said they were conducting a search 900 miles east of Cape Cod in collaboration with the Canadian armed forces and commercial vessels 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 said.

Rescuers have until Thursday before oxygen runs out on the vessel.

– What is the latest on their condition?

A distress signal from the submarine has been sent out, according to oceanologist Dr Simon Boxall of the University of Southampton.

He told PA: “This is second-hand knowledge but my understanding is that they have received a signal from the submarine.

“You can’t use radios underwater. You rely totally on ‘pings’. What they have is really limited communication.

“Apparently they have had, and I don’t know when… they have had an emergency ping saying the vessel is in distress. I don’t know if that is automatically generated or generated by people on board.

“It could be that the vessel is lost already or it could be automatic.”

He said he did not know when the message was transmitted.

– What problems could rescuers face?

Dr Boxall said rescuers would have to first locate the submarine which is not a “trivial ask” as it could be as much as 4km deep, while the boat is beyond the reach of helicopters.

Very few vessels can reach the sort of depths that may be needed to carry out a successful rescue mission, he added on GB News.

If the submarine is found, crews would then have to work out how to get it to the surface.

The worst scenario would be if the hull was crushed, he added.

– What kind of vessel was it?

The 6.7m (22ft) craft weighs 10,432kg (23,000lbs) and is capable of diving to depths of 4,000m (13,120ft) “with a comfortable safety margin”, according to operator OceanGate.

In a May 2021 court filing, OceanGate said the Titan had an “unparalleled safety feature” that assesses the integrity of the hull throughout every dive.

At the time of the filing, Titan had undergone more than 50 test dives, including to the equivalent depth of the Titanic, in deep waters off the Bahamas and in a pressure chamber, the company said.

During its 2022 expedition, OceanGate reported that the submersible had a battery issue on its first dive and had to be manually attached to its lifting platform, according to a November court filing.

– What was the boat’s mission?

The goal of OceanGate’s expeditions has been to chronicle the Titanic’s deterioration as well as the underwater ecosystem that shipwrecks often spawn.

What is left of the Titanic is slowly succumbing to metal-eating bacteria that consume hundreds of pounds of iron a day.

Holes pervade the wreckage while the crow’s nest is already gone. Some have predicted the ship could vanish in a matter of decades as holes yawn in the hull and sections disintegrate.

The company outfitted the Titan with high-definition cameras and multi-beam sonar equipment.

Charting the wreck’s decomposition can help scientists predict the fate of other deep-sea wrecks, including those that sank during the world wars.

Another focus is the sea life as hundreds of species have only been seen at the wreck.

– What are the common misconceptions about the operation?

Dr Boxall said one common misconception is that people will get decompression sickness, commonly nicknamed “the bends”, if the boat moves up too quickly.

He told PA that those on board would not suffer “ill-effects” because the boat is at atmospheric pressure.

He also said it is a mistake to believe passengers could use escape hatches because if they did they would be crushed.

He added on GB News that the boat is not above the surface because if this was the case crews would be able to use radios.