Two of those in the Titan - Paul-Henry Nargeolet and Hamish Harding - are both members of the Explorers Club, a century-old international organisation involved in scientific explorations.

Overnight, there's been an update from its president, Richard Garriott.

Writing on social media, external, he thanked its members for their "support and hard work on the rescue operations for our friends aboard Titan".

"We continue to come together for our friends, their families and the ideals of the Explorers Club, and the cause of safe scientific exploration of extreme environments," he wrote.

"There is good cause for hope, and we are making it more hopeful."

Image source, Getty Images

A notable marine biologist and oceanographer says the incident should be a learning experience for the industry.

David Mearns is nicknamed 'the shipwreck hunter', and has helped solve a number of major maritime mysteries.

Speaking to the BBC, he says: "I certainly feel that now an investigation obviously should happen."

"This sort of thing we cannot allow to happen, and my industry needs to look in on itself and reflect on bringing passengers to such remote locations and such great depths because if things go wrong, there are very, very few options to make a recovery."

He says in hindsight, the fact the Titan was not safety certified is a "cause for concern".

"Would I choose a vessel without a classification? It’s not even allowed. I think that answers that," he adds.

Rebecca Morelle
Science editor

A remotely operated vehicle (ROV) from UK company Magellan is currently at Jersey Airport and due to be flown to the search site later today.

Their sub – called Juliet – recently scanned the Titanic wreck, producing a 3D view of the entire ship.

Once it leaves, it will take about 48 hours to get to the site – which is beyond the timeline given for air for the passengers.

It will be able to dive the full depth of the site, and the team has a detailed knowledge of the deep sea area having been there recently.

The sub has been ready to leave since earlier in the week but has been held up by permissions.

Deep-sea explorer Dr David Gallo believes it would take a miracle to rescue those trapped in Titan, but he remains optimistic.

He told ITV's Good Morning Britain that the noises coming from underwater are "credible and repeatable" meaning teams need to assume they are coming from the submersible and move quickly to locate it.

"We have to, at this point, assume that that's the submarine and move quickly to that spot, locate it and get robots down there to verify that is where the submarine is," he said.

"They've got to go fully ready as if that was the sub because to locate it and get it up to the surface - it takes hours."

Image source, US Coast Guard

We're working on deciphering this graphic. It's from the US Coast Guard, and shows how it has doubled the area it is searching in to around 14,000 square miles.

It appears to show the area above the Titanic shipwreck, marked with a large red X, and the areas being surveyed by ROVs, vessels and planes marked out in neat grids.

We heard earlier from Dr Ken Ledez - a hyperbaric medicine expert - about what could happen as oxygen supplies on the Titan dwindle - you can watch him talking about it by clicking on the play button above.

He says that running out of air isn’t the only danger now.

The vessel may have also lost electrical power, which plays a pivotal role in controlling oxygen and carbon dioxide levels.

As oxygen levels fall, the proportion of carbon dioxide being breathed out by those inside will rise, causing potentially fatal consequences.

"As levels of carbon dioxide build up, then it becomes sedative, it becomes like an anaesthetic gas, and you will go to sleep," Dr Ledez tells the BBC.

Hypothermia - where the body gets too cold to function - is another risk.

Read more about the risks here.

Emma Owen
Live reporter

Welcome - a new team is in place to bring you the latest on the search for the Titan submersible.

The search is now entering a critical phase as it's been estimated that the air supply for those on board will run out within hours.

The US Coast Guard has doubled the area it's looking in as it's thought undersea currents could have dragged the Titan away from the area it went down, and a French ship with a robot that can easily reach the seabed nearly four kilometres down is joining the search too.

Stick with us.

• Onboard, the pilot steers with a modified video game controller
• The sub is extremely narrow, measuring 670 cm x 280 cm x 250 cm (22ft x 9.2ft x 8.3ft)
• There is a private toilet, where a small curtain is pulled across when it is in use
• Before the sub begins its dive, a support team locks the crew inside by closing the hatch from the outside and sealing it shut with 17 bolts

If you’re just joining us here is where rescue efforts stand:

• The US Coast Guard has estimated oxygen could run out for those on board in the next several hours, based on previous estimates

• Rescue officials say they have to remain "optimistic and hopeful" - 10 extra ships and several remote submarines will join the search today, more than doubling operations
• But the captain leading the search has also said: "We don't know where they are, to be frank with you"
• The search site has expanded because of sounds detected on Tuesday and Wednesday. It now covers an area twice the size of Connecticut and a sub-surface area 2.5 miles (4km) deep
• It's not clear whether the noises came from the sub

• The Polar Prince - the research vessel the Titan was launched from - will remain the command centre for the search, sitting near the Titanic wreck site
• Camera-equipped remote-controlled vehicles (ROVs) will be scanning the depths of the seafloor throughout the day.

Derek Cai
Reporting from Singapore

Image source, Twitter

Wendy Rush is the wife of Stockton Rush, the OceanGate boss who is on the missing submersible.

She is also the great-great-granddaughter of Isidor and Ida Straus, external - two first-class passengers who were onboard the Titanic when it sank in 1912.

Ms Rush, nee Hollings Weil, married Stockton Rush in 1986, according to their wedding announcement in the New York Times, external.

She is the communications director at OceanGate and has been on three expeditions to the Titanic wreck, her LinkedIn page says.

The Strauses were among the Titanic's richest passengers. Isidor and his brother Nathan were co-owners of Macy's department store.

Survivors recalled seeing Isidor refuse a seat on a lifeboat until all women and children had got on. Ida, his wife of 40 years, refused to go without her husband. The two were seen in embrace as the ship went down.

A fictionalised version of this scene was depicted in the 1997 film, showing a couple hugging each other in bed as water rises around them.

According to New York Times archives, Isidor's body was recovered at sea around two weeks after the sinking. Ida's remains were never found.

Alan Estrada, who had been on a previous expedition, describes the protocol in place in the event communications are lost.

Rescuers are now searching a sub-surface area approximately 2.5 miles (4km) deep to find the missing Titan.

It’s a harsh and unforgiving environment, more similar to outer space than life on earth.

The Titanic lies within a region called the “midnight zone” - known for its freezing temperatures and perpetual darkness.

People who have taken part in previous expeditions on the Titan have described descending for more than two hours in pitch black conditions, before finally hitting the ocean floor suddenly.

The submersible’s lights offer it a limited line of sight, but nothing beyond a few metres.

First responders will now have to battle these conditions as the search continues.

Tom Housden
Reporting from Sydney

Image source, Reuters

Former US Navy nuclear submarine commander David Marquet warns that hopeful sounds detected in recent hours may not be coming from the Titan submersible.

“I don’t think the noise is them, it could just be natural sounds. We’re hearing noises and more ships are coming into the area, and then we’re hearing more noises, and I don’t think that’s a coincidence,” he tells the BBC.

Capt Marquet believes the odds of survival for those on board are low but have risen slightly, because the equipment needed to raise the Titan is now on its way to the area.

“It’ll be desperately close because it needs to be found before then,” he adds.

A former US Navy diver says there’s still cause for hope as the search for the missing submersible continues.

"Everybody is focused on the 96-hour window for the life support that they're giving to the crew but that's not a hard and fast number and I know that the search teams are not focused on that being a hard and fast number," Captain Bobbie Scholley told BBC Newsnight.

"This will continue to be a rescue mission even past that number, if it has to go past that number, so that gives me hope also."

Image source, PA

1985 - Titanic site discovered by American-French team

1986 - Submersible Alvin explores wreck

1987 - First salvage expedition collects 1,800 Titanic artefacts

1995 - James Cameron visits the wreck - footage is used in his film Titanic

1998 - First tourists dive there

1998 - Section of the Titanic hull is raised

2005 - Two crewed submersibles dive to the wreck

2010 - Autonomous robots map the site

2012 - Wreck now protected by Unesco

2019 - DSV Limiting Factor sub makes five dives

2020 – US and UK agree treaty to protect Titanic wreck

2021 – OceanGate makes first successful attempt to reach the site

2023 – First full-size digital scan of the wreck is created using deep-sea mapping

A Canadian P-3 aircraft detected underwater noises in the search area for the Titan.

The P-3 Orion spotter plane can perform reconnaissance over extended periods. It has infrared and long-range electro-optical cameras, as well as special imaging radar.

Image source, NOAA via SPL

Another asset of the rescue efforts are ROVs - remotely operated vehicles.

As well as assisting in the search effort, ROVs could attach cables to the sub, should it be located, to help take the vessel to the surface.

Another piece of equipment is the Victor 6000 - an unmanned French robot which can dive up to 6,000 metres.

It has arms that can be remotely controlled to cut cables or otherwise help release a stuck vessel.

But it doesn't have the capability of lifting the submersible on its own.

Image source, Ifremer

The US Coast Guard says more noises have been heard in the search for the missing Titanic submersible. They still don’t know what these noises are but rescue teams are looking in the area.

The surface search for OceanGate’s Titan sub has been expanded and is now twice the size of the US state of Connecticut. The sub-surface search is now 2.5 miles (4km) deep.

Royal Canadian Airforce planes are among the latest additions to the expanded search. More ships and Robotically Operated Vehicles (ROV) are also joining the mission.

The Coast Guard says it is still in search and rescue mode and remain hopeful the five people aboard the sub will be found.

However, the crew are now thought to have less than 20 hours of oxygen left. This is based on an earlier estimate of Titan’s life preserving capabilities.

Image source, NAVSEA

It may look like a giant coil of rope, but what it can do is quite extraordinary.

This is the Flyaway Deep Ocean Salvage System (FADOSS), and it is used by the US Navy.

There's one FADOSS currently at St John's and is waiting for an available ROV (remotely operated vehicle) so it can be deployed to the search area.

A US Navy official said FADOSS works by welding the system to the deck of a ship and then binding the other end of the line to an ROV, which dives into the ocean depths.

It's strong enough to lift 60,000 pounds (27,000kg) and it can go as far down as 20,000ft (6096m) into the sea.

The deepest recovery effort recorded with a FADOSS is 19,750ft. The Titanic wreck is at a depth of 12,467ft.

Usually, FADOSS is used to recover large, bulk and heavy sunken objects like aircraft and small boats.

Our reporter Eloise Alanna has spoken with Dr Ken Ledez, a hyperbaric medicine expert at Memorial University in St John’s, Newfoundland.

Dr Ledez explained some of the factors at play for those onboard the missing Titan sub.

He says surviving after oxygen runs out will depend on a person's metabolism and some may survive longer than others.

"It’s not like switching off a light, it’s like climbing a mountain," he said.

"They're going to do everything they can to reduce their oxygen consumption, they're going to rest, they're going to try to be as relaxed and calm as possible."

Too much activity could cause an increase in metabolism and create more carbon dioxide, he explained.

Dr Ledez also says "hypothermia could be their friend".

"There is a possibility if they cool down enough and lose consciousness they could live through it... The heart beat can be really slow when cold," he said.

"Could they go on a week after the limits? I doubt it. But some may survive longer than others."