Today's hearing at the US Coast Guard inquiry into the Titan sub implosion has ended, and we're going to pause our live stream coverage now.

Wednesday's session saw analysis of the wreckage of OceanGate’s submersible which revealed how its hull came apart.

Images of the craft's pieces scattered across the sea floor show the carbon fibre hull had separated into multiple layers - a known problem with the material. It was not confirmed that the hull was the first part of the sub to fail, but the details suggest it is a key focus of the investigation.

You can read about this evidence and more in our news story here.

There's just two more days left in the inquiry, which wraps up on Friday, and our science team will be on hand to bring you updates as more testimonies are heard. Thanks for joining us.

The Coast Guard hearing has taken a break for lunch. When they come back, we are expecting more testimony from William Kohnen, whose company Hydrospace designed a window used on the Titan sub.

Then the investigation panel will hear from marine engineer Bart Kemper.

We will end our live text updates for now, but you can continue to follow the hearing by watching it live at the top of this page.

Catch up below on what we've learned about the Titan's 2023 fatal dive:

'All good here': Titan sub's last messages before implosion

Titan sub broke days before doomed dive, says science chief

Crew's final words and new footage of wreck: Key takeaways from Titan sub hearings

Correction 30 September 2024: This post was updated to clarify that HydroSpace did not build the window on Titan at the time of the implosion.

Richard Gray
Editor, BBC Future

Image source, Reuters

The wreck of the Titanic lies in total darkness on the seabed, nearly 2.4 miles (3.8km) below the surface.

It is an incredibly hostile environment that makes visiting the wreck a challenge.

Travelling that deep can take time – it is a two-hour-long descent. Below about 3,300ft (1,000m), sunlight is unable to penetrate the water and everything exists in what is aptly named the "midnight zone". It makes it easy to become disorientated without accurate navigation techniques.

As a submersible travels downwards, the pressure exerted on it by the water above also increases. At the depth of the Titanic wreck, the pressure is about 390 times greater than we experience on the surface.

Submersibles traveling to that depth need to be able to withstand these huge forces, and usually have thick metal hulls to do this.

• Read more about the treacherous waters around the Titanic wreck here

Today's hearing into the Titan submersible disaster featured testimony by William Kohnen, CEO of the US engineering group Hydrospace and from Don Kramer, an engineer from the National Transportation Safety Board. Here's what they've told the inquiry on Wednesday:

• NTSB engineer Don Kramer told investigators about a 2022 incident in which a loud bang was heard as the Titan was resurfacing after a dive. It could have been some kind of change occurring in the carbon fibre fabric of the vessel's hull, he said.
• The sub's hull seemed to behave differently after that 2022 dive.

• William Kohnen's company Hydrospace had designed a window requested by OceanGate for Titan, but had made it clear that it could not be rated for deep-sea diving without significant testing.
• Hydrospace told OceanGate that it could only be certified as a flat window, which is not suitable for extreme depths.
• The head of a marine classification firm based in Germany told Kohnen that in 2018 or 2019 they had offered OceanGate to conduct a review setting out steps to ensure that Titan could be certified and that the offer had been flatly rejected.

Caitlin Wilson
Live editor

Kohnen says his company Hydrospace and Titan parent company OceanGate had an open line of communication to talk about submersibles.

Hydrospace told OceanGate "book, chapter and verse" of what they needed to do to test the non-standard window built for the Titan, he says.

Until that testing was complete, Kohnen says, the "hybrid window" OceanGate requested could not be rated for deep-sea diving. It could only be certified as a flat window, which is not authorised for use in extreme depths.

"That hump on the outside? It wouldn't hurt anything... but it is just a flat window," Kohnen explains.

But OceanGate ultimately did not test the window as suggested, Kohnen says, though they did not explain why.

"Something happened at OceanGate," he says. Previously, there had been "a lot of conversation about the testing they were going to do."

"Somewhere they changed their mind."

When Hydrospace delivered the window, "It was like 'What do you mean you're not - you need to be doing the testing,'" Kohnen recalls telling OceanGate. "'This is not optional.'"

"It was left as a hanging chad, like 'We'll see,'" Kohnen says he remembers about that interaction.

Caitlin Wilson
Live editor

Image source, USCG

William Kohnen is now telling the inquiry about the window on the Titan sub.

Kohnen helped to build the window for the Titan as part of the Hydrospace Group company.

He also worked with the American Society of Mechanical Engineers Committee for Pressure Vessels for Human Occupancy, also known as the ASME PVHO.

Kohnen is a member of the group's safety standards committee, which he calls "important" work to certify underwater vessels.

According to Kohnen, an "arched" acrylic window would be structurally sound and is what is usually recommended for underwater vessels. But he says OceanGate wanted the visual benefits of a flat window, so that passengers could better view the sea outside. They suggested a "hybrid" flat-round window, he says.

"It's worth looking into it. It's an idea. We don't know. It's a non-standard window," Kohnen recalls telling OceanGate.

"You'd have to do testing for it, but it's worth trying out. If it's important for your business, figure it out," he says.

Hydrospace designed the window OceanGate requested, but Kohnen says that he told them the PVHO committee was not able to certify it since it was "non-standard".

"You need to do a bunch of testing" to make sure such a window is safe, he says. He says standard procedures for testing that kind of window exist.

The fatal trip the Titan set out for in June 2023 was intended to take its passengers down to view the wreck of the Titanic at the bottom of the Atlantic Ocean, where it has sat since it sank crossing from Europe to New York in 1912.

The graphic below shows just how deep below the surface the Titan aimed to dive in order to reach the ship.

The US Coast Guard has been hearing from experts and former OceanGate employees since 16 September. Here are some of the key points that have have emerged from the inquiry so far:

• Earlier on Wednesday, the hearing heard from NTSB engineer Don Kramer, who told investigators about a 2022 incident in which a loud bang was heard as the Titan was resurfacing after a dive and could have been some kind of change occurring in the carbon fibre fabric of the vessel's hull.

• During testimony last week, Former OceanGate operations director David Lochridge said he had highlighted safety issues during a meeting in 2018 - before being fired. He said he believed the company had "bypassed" rules and that an incident involving Titan - which was not certified as seaworthy as it was operating in international waters - was "inevitable"
• A transcript of that 2018 meeting captured a heated exchange between Stockton Rush - the head of OceanGate who died during the accident - and Lochridge. Rush said: "I think this is one of the safest things I will ever do."
• Patrick Lahey, co-founder of submersible manufacturer Triton, told investigators he wasn't impressed by Titan when he visited it. He stressed the importance of certifying submersibles through a process that involves an extensive safety assessment by independent marine organisations.
• Investigators revealed one of the crew's final messages before it lost contact with a ship above water: "All good here."

Investigators are now hearing from Wednesday's second witness, deep sea engineer William Kohnen.

He was among the maritime experts who signed a 2018 letter addressed to OceanGate CEO Stockton Rush warning him about the possible dangers of the Titan sub.

Stick with us, and remember you can watch the hearing live at the top of this page.

Rebecca Morelle and Alison Francis

Forensic experts will have examined Titan’s wreckage to find the root of the failure. There were several issues with its design, some of which Don Kramer went through in his testimony just now.

The viewport window was only rated to a depth of 1,300m (4,300ft) by its manufacturer, but Titan was diving almost three times deeper.

Titan’s hull was an unusual shape, it was cylindrical, rather than spherical. This means the effects of the crushing pressure of the deep were not distributed equally.

The sub’s hull was also made out of carbon fibre - a highly unconventional material as its considered to be unreliable in the deep water.

With every dive, the carbon fibre would have been compressed and damaged, becoming weaker every time the sub went down.

Kramer explained just now how the hull began behaving differently after a loud bang was heard on a dive in 2022.

Another concern was the junctions between different materials: the carbon fibre was attached to two rings of titanium creating weak points.

We don't yet know - and Kramer did not say - exactly which part of the sub failed first to cause the catastrophic implosion in 2023.

Rebecca Morelle
Science editor

Don Kramer has also provided data collected from sensors on the sub that measured how the hull - the part where the passengers were - was responding to strain.

Throughout the hearings the US Coast Guard has repeatedly asked witnesses about a loud bang that was heard in 2022 as the sub was coming back to the surface.

At the time Stockton Rush said he thought it was the sub shifting in the metal frame that surrounded it.

But Kramer said that the sensors picked up this event - and data from subsequent dives showed that the sub was responding differently to strain.

This suggests the bang was in fact some kind of change occurring in the fabric of the hull.

Image source, US Coast Guard

Rebecca Morelle
Science editor

Dr Don Kramer, an engineer at the National Transportation Safety Board, has been showing us images of the wreckage of Titan on the seafloor.

He focussed on pieces of the hull - this is the part of the sub that was supposed to keep the passengers safe from the pressure.

It was made from carbon fibre and much was destroyed, but some parts were scattered across the sea floor.

The images show how the layers of carbon fibre are coming apart - a process known as delamination.

There were substantial delamination close to where the carbon fibre was attached to the titanium sections of the sub.

He hasn’t said that the hull was the first part of the sub to fail - but his focus on this material suggests its key to their investigation.

Rebecca Morelle
Science editor

Image source, OceanGate

Dr Don Kramer an engineer at the National Transportation Safety Board (NTSB) is giving evidence, which you can watch live at the top of this page.

He’s been outlining problems with the Titan’s hull - the part where the passengers would have been.

It was made from layers of carbon fibre mixed with a resin - we’ve heard repeatedly from the hearing how this was an extremely unusual material for a deep sea subs.

Kramer detailed how his team had analysed samples of Titan’s hull, trimmings left over from when the sub was made.

He detailed how there were places where the layers were coming apart - a process known as delamination.

Closer inspection showed wrinkles, voids and gaps.

Any irregularities in the material would have affected how the hull performed under the immense pressures of the deep.

Rebecca Morelle
Science editor

The US Coast Guard have heard about significant issues with the Titan’s design.

They include concerns about the viewport window was only rated to a depth of 1,300m (4,300ft) by its manufacturer, but Titan was diving almost three times deeper.

Titan’s hull was an unusual shape, it was cylindrical, rather than spherical. This means the effects of the crushing pressure of the deep were not distributed equally.

The sub’s hull was also made out of carbon fibre - a highly unconventional material as its considered to be unreliable in the deep.

With every dive, the carbon fibre would have been compressed and damaged, becoming weaker every time the sub went down.

Another concern was the junctions between different materials: the carbon fibre was attached to two rings of titanium creating weak points.

Rebecca Morelle
Science editor

On Tuesday Amber Bay, OceanGate’s Director of Administration, provided an insight into OceanGate’s finances.

The cost of the sub’s development and build, and later its operations, were covered by investors as well as money from passengers who were paying for dives to the Titanic.

In 2021, a place on the Titan was marketed at about $150,000 but after 2022 that rose to $250,000 per person. Bay said this was to cover the cost of taking the support ship and expedition crew out to the middle of the Atlantic for the dives.

She said that if a customer didn’t get to see the Titanic, then no refunds were offered - but the passenger was invited to try again on another expedition.

This created a backlog and added to costs. So the dives were then being funded by deposits from the next group of passengers, suggesting cash flow was a problem. Bay also said she and Rush had delayed their salary payments when funds were short.

The US Coast Guard asked whether there was a desperation within the company to complete the dives.

Bay replied that the company wasn’t desperate, but that “there was definitely was an urgency to deliver on what we had offered”.

On Monday Phil Brooks, OceanGate’s former director of engineering, said the company was strapped for cash - and this was leading to safety shortcuts.

"It was clear that the company was economically very stressed, and as a result that they were making decisions and doing things that I felt the safety was just being compromised way too much."

Rebecca Morelle
Science editor

On Tuesday, the US Coast Guard heard evidence from Karl Stanley, the owner of a deep-sea diving expedition company.

He'd joined OceanGate CEO Stockton Rush on a dive in the first prototype of Titan in April 2019. This descent took place in the Bahamas and the sub reached 3,760m.

The Coast Guard heard that before the dive the prototype sub had been struck by lightning.

But Stanley said he'd not been told about this event, and he would not have got in the sub had he known.

He said that he heard worrying noises during the descent.

"We got down there and it’s making noises - the sound of a carbon fibre bond breaking... We were all clearly a little disnerved.”

He went on: "Then there was kind of like a grand finale of cracking sounds as we were getting close to the surface."

The sub made some more dives after this - although not with Stanley on board.

In June 2019, an OceanGate sub pilot spotted a large crack on the internal surface of the carbon fibre hull.

After that, the prototype did not dive again. Instead a new hull was built and the prototype’s other components were attached to it. This was the final version of Titan.

After learning about the problems with the hull, Stanley told the hearing that he believed that "OceanGate came very, very close to killing me."

Image source, Dawood family/Lotus Eye Photography/Reuters

Five people perished when the Titan submersible imploded last year.

The men on board the sub included Stockton Rush, the 61-year-old CEO of OceanGate, British-Pakistani businessman Shahzada Dawood, 48, and his son Suleman, 19, and British businessman Hamish Harding, 58.

The fifth man on board, Paul-Henri Nargeolet, was a 77-year-old former French navy diver and renowned explorer who had been to the Titanic dozens of times.

Dive expert David Mearns said he lost two friends Harding and Nargeolet, in a "most horrific way". OceanGate called the group of men “true explorers”.