After several days of searching in a remote area of the North Atlantic, the five people aboard a submersible near the Titanic wreck have been declared dead.
The cause of their deaths was a "catastrophic implosion of the vessel", according to US Coast Guard Rear Admiral John Mauger.
Here's what an implosion means, how it happens and whether the passengers could have felt it.
An unmanned deep-sea robot deployed from a Canadian ship discovered the wreckage of the Titan submersible on Thursday morning.
Soon after the debris of the vessel was found, OceanGate Expeditions, the US-based company that operated the submersible, released a statement confirming the deaths of the five passengers.
"We now believe that our CEO Stockton Rush, Shahzada Dawood and his son Suleman Dawood, Hamish Harding, and Paul-Henri Nargeolet, have sadly been lost," the company said.
Five major fragments of the Titan were located in the debris field left from its disintegration.
This included the vessel's tail cone and two sections of the pressure hull, Coast Guard officials said.
"The debris field here is consistent with a catastrophic implosion of the vehicle," Mauger said.
No mention was made of whether human remains were sighted.
Wreckage of the Titan submersible was found 488 metres from the bow of the Titanic wreckage.
This was 4 kilometres below the surface.
The US Coast Guard believes the nature and location of the debris suggests a catastrophic implosion which would have killed all five people on board.
The US Coast Guard said it was too soon to say when the implosion happened.
The Titan had been missing since June 18.
He said it was not detected by sonar buoys used by search crews, which suggests it happened before they arrived.
But one source says the implosion may have happened days ago.
A US Navy acoustic system detected an "anomaly" on Monday, a senior military official told The Associated Press today.
It found the anomaly to be "consistent with an implosion or explosion in the general vicinity of where the Titan submersible was operating when communications were lost".
The official spoke on condition of anonymity to discuss a sensitive acoustic detection system.
The Navy passed on the information to the Coast Guard, which continued its search because the Navy did not consider the data to be definitive.
The Navy's possible clue was not known publicly until today, when The Wall Street Journal first reported it.
Implosion is an explosion in reverse, according to Professor Stefano Brizzolara, the co-director of Virginia Tech Center for Marine Autonomy and Robotics.
"Imagine a cylinder; during an explosion, the charge placed in the centre, ignites and causes the pressure at the centre to increase instantaneously," Professor Brizzolara told ABC News.
Such intense pressure is what causes the ejection of mass to move from the centre of the cylinder to the outside at an "incredible speed".
"An implosion is the reverse of this," Professor Brizzolara said.
"The inside of the cylinder can practically be considered void, while the pressure of the wave that breaches the hull is 400 times larger."
"This causes a violent flow of water from outside of the cylinder to the inside, with incredible speed."
"We're talking water rushing in at a speed of the order of 1000km."
"Catastrophic implosion" literally means breaking into pieces and small fragments, says Professor Brizzolara.
Here's where "the hull" comes in - it's the main body of a ship or vessel and it's made from carbon fibre and titanium.
"Catastrophic is referred to the way the carbon fibre reinforced plastic hull collapses when the hull implodes."
Carbon reinforced plastic, instead, collapses "catastrophically", says Professor Brizzolara.
"This is because the material is not ductile as metal alloys and therefore it 'catastrophically' implodes."
For context, navy submarines use high strength steel or titanium alloys, Professor Brizzolara says.
"This is why current rules and regulations do not consider composite materials for underwater vehicles meant to operate at large depths," he said.
A flood or a failure of the pressure vessel would have likely caused the implosion, says submarine expert Eric Fusil from the University of Adelaide.
That kind of "catastrophic event" would have happened within 20 milliseconds, Professor Fusil told ABC News Breakfast this morning.
The Titan's pressure hull was made of a combination of titanium and a composite material of carbon fibres, which he described as "very new".
"The titanium pressure vessel is very elastic - it can crush and then restore its initial shape," Professor Fusil explained.
"But the carbon fibres are completely different - it's something very stiff."
"We have those two opposite forces," he said.
Professor Fusil said it's an "experimental technology" and it was too early to tell whether that design caused the issues.
The five passengers inside the Titan submersible might not have realised it was even happening, Professor Fusil says.
"They wouldn't have realised they were dying because they cannot process that information that quickly," he said.
Implosions can be similar to a balloon, says forensic engineer Bart Kemper.
"When I take a needle and poke it into a balloon, once you break that balloon, it's gone," Kemper told 7.30's Sarah Ferguson.
"That's exactly the problem you have with a pressure vessel, and the fact that this is external pressure, not internal pressure, it doesn't matter.
"Once you lose integrity, with these kind of pressures, it's gone," he said.
In 2018, submarine experts had warned the company operating the Titan, OceanGate, that without industry oversight the submersive was exposing passengers to possible catastrophic failure.
Underwater sounds described as "banging noises" by a Canadian surveillance vessel initially sparked hope for a possible rescue.
But these underwater sounds heard Tuesday and Wednesday were probably unrelated to the submersible, The Associated Press reports.
The sounds in the Titan search were picked up using devices called sonobuoys, which can be tossed out of aeroplanes to detect noises, to avoid interference with ship sounds, said Matt Dzieciuch, an ocean acoustics expert at the Scripps Institution of Oceanography.
The ocean is a "noisy place," Dr Dzieciuch said.
There are many other potential sources of sound underwater, including from fish, other animals and of course human-made instruments, he explained.
Search teams heard the banging noises at 30-minute intervals, US Coast Guard Captain Jamie Frederick said at a press conference.
But Mr Frederick went on to say analysis of the noises had been "inconclusive".
"With respect to the noises specifically, we don't know what they are, to be frank with you."
OceanGate has been chronicling the Titanic's decay and the underwater ecosystem around it via yearly voyages since 2021.
The company has not responded to additional questions about the Titan's voyage this week.
The Coast Guard will continue searching for more signs about what happened to the Titan.
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