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News that noises have been picked up in the hunt for a missing submersible has offered a glimmer of hope that the five men on board are alive.
The sounds were recorded by sonar buoys in a massive rescue operation, which is racing against time to find the Titan in the middle of the Atlantic Ocean. It was lost on a deep-sea voyage to the Titanic wreck three days ago.
Underwater operations have been relocated to investigate the noises, the US Coast Guard has said, but so far they haven't found anything.
And with oxygen supplies expected to run out at about 10:00 GMT on Thursday, the next few hours are critical.
The US Coast Guard says noises were heard on Tuesday and Wednesday but it does not yet know what the noises are. It says the US navy is analysing the data, but so far it remains inconclusive.
Some media reports have suggested the noises were heard at 30-minute intervals.
Deep-sea experts who spoke to the BBC say it is hard to determine what these noises might be without seeing the data, and Rear Adm John Mauger - who is leading the search - has also confirmed the source of them is unknown.
But it is possible they could be short, sharp, relatively high-frequency noises, made from within the sub by hitting a hard object against the end of it.
Frank Owen, from the Submarine Institute of Australia, says he is confident - based on the information available - the sounds are coming from inside the vessel.
"If there was a 30-minute interval, it's very unlikely to be anything but human related," he told the BBC.
The men on board include British businessman Hamish Harding, 58, British-Pakistani businessman Shahzada Dawood, 48, his son Suleman, 19, and Stockton Rush, 61, the chief executive of OceanGate, which runs the voyages at a cost of $250,000 (£195,270) per head.
But Mr Owen says the noises "smack of advice" coming from the fifth man inside - 77-year-old Paul-Henry Nargeolet, a former French navy diver and renowned explorer.
"He would know the protocol for trying to alert searching forces… on the hour and the half-hour, you bang like hell for three minutes," Mr Owen said.
Banging on the vessel's hull every half hour is standard naval protocol around the world for stricken vessels aiming to send word of their location to rescuers, Mr Owen told LBC News.
The decision to relocate the search indicates authorities are thinking similarly.
But in previous maritime searches - like those for missing Malaysian Airlines flight MH370 in 2014, and the Russian submarine Kursk in 2000 - underwater noises were heard too, and yielded no results.
And Rear Adm Mauger has said there are a lot of metal objects at the Titanic site that could have been causing the noises.
More on the Titanic sub
The other ray of hope is that these sounds were picked up by the sonar buoys - listening devices dropped from aircraft or ships - at all, Mr Owen says.
The Titanic lies 12,500ft (3,800m) beneath the ocean surface, where the buoys sit.
All forms of electromagnetic radiation, including radio and radar, are mostly useless underwater, but sound can travel fast over great distances.
It is possible that noises from deep ocean layers could get through to the sonar buoys, Mr Owen says, but it is more likely that the sounds are coming from the same ocean layer.
"It is very difficult to hear noise below the [top] layer because the sound gets refracted by this drop in temperature.
"But when it's in that isothermal layer... between the surface and 180m... the sound behaves really quite straight."
Mr Owen says if the sounds are indeed coming from the sub, rescuers should be able to locate it pretty quickly.
"[They can] lay a pattern of buoys around that area, so they can get cross-bearings."
"The sonar buoys' receiver is able to plot that sort of information really very quickly... it would take a very short time to find."
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Experts say that if one sonar buoy has picked up the sounds, they could locate them to about 1-2km (0.6-1.2 miles). And if more have picked them up, it could be possible to narrow the search area down to 100m.
However, the underwater vehicles which have been sent to find the origin of the noise have so far not found anything, the Coast Guard said at a news conference on Wednesday.
There are two remotely-operated vehicles (ROVs) currently searching and several more were expected by Thursday morning, the Coast Guard said.
It would be what happens after the sub is located that could really slow things down, experts say.
How deep the vessel is or what problems it faces are both still unknown.
"It is quite possible that the submersible is actually on the surface somewhere and hasn't yet been found," David Russell - a former Royal Navy officer who helped search for the Kursk - told the BBC.
If the sub is in the top layer of the ocean, a retrieval could be fairly quick.
But if it is at deeper levels, and needs help resurfacing, that would be a complex task.
"We're then talking about bringing it back to the surface using some form of sophisticated drones to attach wires and ropes to the vessel, then to pull it up or put it on to a crane," Mr Russell said.
"We've done that kind of thing before, though never - as far as I'm aware - at this kind of depth."
But an operation like that would probably require equipment that Mr Owen says isn't available in the area yet.
US and Canadian agencies, navies and commercial deep-sea firms are all helping the rescue operation, which is being run from the US city of Boston in Massachusetts.
Scouring the search zone are military planes, a submarine and a number of ships - some equipped with underwater ROVs (remotely operated vehicle).
However, those are more useful for gathering information rather than in the event of any retrieval, Mr Owen says.
"You need heavy duty equipment to bring something up like that," he adds.
"Even the bringing the extra cable down that you're going to hook on needs a huge winch of its own, because you're talking about probably 5-6km of wire."
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