Image source, Reuters

Titanic expert Tim Matlin told Reuters news agency that the floor of the Atlantic Ocean - the Titanic's last resting place - is "pitch black and freezing cold".

In addition to this, "the seabed is mud and it's undulating. You can't see your hand in front of your face. The only way you can find where you are is by a thing called sonar. Not even radar works," he said.

Matlin added: "So it's really a bit like a moon shot. It's really a bit like being an astronaut going into space. So I do fear for the lives of those explorers who are on board."

What value is there in exploring the wreck?published at 15:07 British Summer Time 20 June 2023

15:07 BST 20 June 2023

Patrick Hughes
Science reporter

Chikwanda Kambole from Zambia wants to know what the value of exploring the wreck of the Titanic is, and are the risks worth it?

There's been a lot of focus on the cost of being part of one of these dives, for the people rich enough to go. But do they bring any other benefits too?

OceanGate claim on their website that their fleet of 5‑person submersibles allow marine scientists to conduct biological, chemical, geochemical, geological and geophysical studies, and that their submersible provides an alternative to the low number of high-cost submersibles and a solution to unmet research needs.

They have performed a number of successful expeditions to the wreck of the Titanic in 2021 and 2022, and have stated their intent to return annually to document the Titanic and its rate of decay.

That includes a survey that would collect high-definition images and video, as well as laser and sonar data to assess how quickly the wreck is decaying.

They also say it would document the flora and fauna inhabiting the wreck site and compare the data with data gathered from prior expeditions to learn how the habitat has changed in the maritime heritage site.

Jessica Parker
Reporting from Boston

The Boston Coastguard is now the centre of a search operation as well as huge interest from media all over the world.

Camera crews are lined up along Boston Harbour as we all wait to hear the latest on efforts to locate the Titan sub.

I understand from officials here that the search is ongoing. But the area they’re looking in is extremely remote – 900 miles from the US east coast and about 430 miles off Newfoundland in Canada.

Changeable weather and poor visibility are likely to be challenges that teams are having to contend with.

How are the crew affected by the air pressure in the Titan?published at 15:00 British Summer Time 20 June 2023

15:00 BST 20 June 2023

Sura Bunchongchit
Submarine expert

We’ll start this question and answer session with Jane, who has been in touch to ask: "How would the crew have to resurface taking into account effects of nitrogen, ie the bends?"

Manned submarines and submersibles have a pressure hull, which is a strong structure to withstand the outside pressure at diving depth.

So if we assume that the pressure hull is still intact, the crew will only experience normal pressure of the atmosphere similar to on the surface.

And the Titan can return to the surface at normal rate.

We've been putting your questions to our science reporter Patrick Hughes and submarine expert Sura Bunchongchit.

Stay with us as they bring you the latest on the submersible and the rescue operation.

Search teams are racing against time to find the tourist submersible, but the rescue operation is frustrated by the vessel's remote location and depth of the waters:

There are still some details missing in the narrative of the missing sub, which we are working hard to fill, but here is a brief timeline of what we know:

• A ship carrying the submersible set off from St John's, Newfoundland on Saturday

• The dive started at about 04:00 EST, according to a Facebook post from Hamish Harding
• One hour and 45 minutes into the dive Polar Prince lost contact with the crew of the Titan sub
• On Monday the US Coastguard confirmed there were five people onboard the missing sub and that the sub went missing more than 24 hours previously

• At a news conference on Monday afternoon, the US Coast Guard's Rear Adm John Mauger predicted there were between 70 and 96 hours of oxygen left
• Throughout Tuesday morning, those reportedly on board the missing sub were named
• The search was expanded to deeper waters in the last hour, according to CNN

Image source, Reuters

Hamish Harding's aircraft firm Action Aviation has put out a statement on behalf of the chairman's family.

Renowned explorer Harding, who has flown to space and holds three Guiness World Records, is onboard the missing sub.

The statement said: "Both the Harding family and the team at Action Aviation are very grateful for all the kind messages of concern and support from our friends and colleagues."

The family also thanked the authorities and companies who have stepped in to help with the rescue efforts.

"The team at Action Aviation are extremely proud of Hamish and we look forward to welcoming him home," it added.

The aircraft firm and Harding's family also asked for privacy at this time.

Foreign Secretary James Cleverly said he hoped the missing submarine and its passengers would be "swiftly found". He was speaking alongside US counterpart Antony Blinken in London, where they met to discuss bilateral relations and the situation in Ukraine.

Cleverly said the thoughts of the government were with "those individuals who are currently in the submersible in the North Atlantic".

"We wish them all the luck, and of course we hope that they will be swiftly found and returned to their loved ones," he said. Blinken said: "I can only echo that, James."

"Time is very short and the next few days will be critical," an oceans expert has told BBC News.

Dr Simon Boxall, University of Southampton lecturer in oceanography, said it was a "real challenge" to identify where the missing submersible is and to get it back.

Describing what the conditions are likely to be in the submersible if it is on the sea floor, he said: "It's going to be hot, it's going to be cramped.

"There is no escape pod. If you came out the water at those depths you'd be crushed, so they're totally reliant on the submersible being found."

He believes its unlikely the Titan is on the surface, and that it may have become snared on part of the wreckage.

"It's an enormous challenge, one we've never had to tackle before," he added. "There have been tragic cases of military submarines sitting on the seafloor where they have a lot more resource and lot more oxygen available, but in this case time is very short and the next few days will be critical."

The company of businessman Shahzada Dawood has said they are praying for the "swift and safe return" of him and his son Suleman.

The Engro Corporation, where Dawood is the vice-chairman, said it was praying for the safety of all onboard and asked for privacy for the family.

"There is limited information available beyond this that we know, and we humbly request that speculation and theorisation is avoided.

"We, at Engro, remain in prayer for their swift and safe return," said the company in a Facebook post.

We're just hearing that authorities are expanding the search for the missing sub into deeper waters.

A US Coast Guard official said work had continued through the night with a range of partners, and the search is now being extended to the subsurface in the area, Rear Admiral John Mauger told CNN.

BBC News has been speaking to marine scientist and expedition leader David Mearns, who has been keeping a close eye on developments off Newfoundland.

He tells us that a commercial pipe-laying ship has arrived in the area where the submersible was last reported, and says this is welcome news.

“Just in the last hour or so... another commercial vessel, a very capable pipe laying vessel, modern ship, with ROV (remotely operated underwater vehicle) capabilities, is actually over the site now and we’re just hoping that it has the capabilities to reach those kind of depths – 3,800 metres – to search for the submersible and have the ability to recover it.

"There is some hope that that could happen.”

The BBC has confirmed esteemed French explorer Paul-Henry Nargeolet is on board the missing sub.

Nargeolet is a former diver for the French navy, and was part of the first expedition to visit the wreck in 1987, just two years after it was found.

Nick-named 'Mr Titanic', he has reportedly spent more time at the wreck than any other explorer.

Family spokesman Mathieu Johann said he hoped that Nargeolet's composure and military career will reassure the crew on board, even if the outcome of the operation does not depend on him.

We're just hearing that Prime Minister Rishi Sunak has said his thoughts are with the families of the people on board the submersible.

Billionaire Hamish Harding, who runs an aviation company, and businessman Shahzada Dawood and his son are among the British citizens onboard.

The PM's official spokesman said: "The families involved will be deeply concerned and his thoughts are with them and the Foreign Office is providing support."

He added Sunak "clearly wants to pass on his thanks to those that are responding" to the situation.

Nadine Yousif
BBC reporter, St John’s, Newfoundland

We've just touched down in St John’s, Newfoundland.

The oceanside city of 113,000 people is located on the furthest eastern tip of Canada’s shore. It's the closest city to the shipwreck site of the Titanic, which lies 600km south-east of here.

Its close proximity to the famous shipwreck, coupled with its position on the treacherous Atlantic waters, has made St John’s a hub for Titanic researchers.

This is where the crew on board the missing submersible first began their journey, travelling out on a Canadian research ship before diving 13,000ft deep into the ocean to explore the Titanic wreck.

A major search and rescue mission has since been sparked off the coast of St John's in an attempt to recover the submersible and the five people inside it.

We'll bring you more updates from here throughout the day.

Claire Marshall
Environment correspondent, BBC News

I went down nearly 1000 ft (300m) in a submarine to the seabed off the coast of the Antarctic peninsula and nearly didn’t make it back to safety.

It was to report on a Greenpeace mission to gather evidence of life to help create a marine reserve.

The dangers were clear: drifting under an ice sheet and losing power, or a fire breaking out. It was a two-person submarine, just me and the pilot.

There was a lever by my feet to release ballast so we would come to the surface in an emergency, and a fire extinguisher tucked behind my shoulder.

Once submerged the pilot checked in every few minutes with the ship, but after an hour at the seabed we got news of a storm at the surface and lost contact with the ship.

We ascended quickly but came up in an ice field; the rescue boat couldn't reach us.

In an incredibly dangerous operation, a diver was put into the water and managed to hook the sub and we were eventually winched back on deck.

Missions like this to the Titanic are rare - and expensive.

Few submersibles capable of taking people 4000 metres below the ocean surface exist. OceanGate, the company behind the expedition, says it is one of the only private companies in the world that has one.

And it has only taken the Titan on a handful of dives to see the famous wreck - their first taking place in 2021, more than three decades after the first ever manned dive in 1987.

OceanGate on its website says its sub is ground-breaking - more cost efficient and lightweight than others - and hopes it will revolutionise deep sea exploration like rockets did for space.

Image source, .

We've been following search and rescue in the North Atlantic, after a tourist submarine on its way to the Titanic wreck went missing on Sunday morning - more than 48 hours ago.

Here's a look at what we know:

• US and Canadian search teams are racing against time to find the Titan submersible - which there so far has been no sign of
• The submersible vanished in the ocean around 900 miles (1,448km) from Cape Cod, Massachusetts at a depth of roughly 13,000 feet (4km)
• Five people are on board, including British explorer Hamish Harding, British-Pakistani businessman Shahzada Dawood and his son. Also the CEO of the submersible's company Stockton Rush and French explorer Paul-Henry Nargeolet
• It is believed the crew members had at most roughly four days of oxygen left as of Monday evening, the US Coast Guard estimated

The BBC has spoken to a submarine expert to work through a number of scenarios for where the missing submersible might be.

One is that it released a "drop weight" after an emergency, in order to bring it to the surface, said Prof Alistair Greig from University College London.

He told BBC Science Correspondent Pallab Ghosh: "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.

Another scenario, he says, is that the hull was compromised resulting in a leak. "Then the prognosis is not good."

If it has gone down to the seabed and can't get back up under its own power, the options - according to Prof Greig - are very limited.

"While the submersible might still be intact, if it is deeper than more than 200m (656ft) there are very few vessels that can get that deep, and certainly not divers. The vehicles designed for navy submarine rescue certainly can't get down to anywhere near the depth of the Titanic."

You can read more here.