The world waits for news as the window of survival for the five passengers onboard the Titan sub is rapidly closing, with the oxygen supply due to run out at 7.08am ET (12.08pm UK time) today, the US Coastguard has projected.
The announcement has painted a bleak picture for those trapped inside the stranded vessel, but officials have continued to insist that the hunt is '100 per cent' still a search and rescue mission.
The Titan, a tourist submersible which runs $250,000 tours of the doomed Titanic ship and is operated by OceanGate Expeditions, has been underwater since 8am on Sunday with five people on board.
The group, dubbed the Titan Five, includes British billionaire Hamish Harding, OceanGate CEO Stockton Rush, French navy veteran PH Nargeolet and Pakistani businessman Shahzada Dawood and his son Suleman.
After embarking on its 12,500ft descent, the sub lost contact with its expedition ship, the Polar Prince, at 9.45am but was not reported as missing to the Coast Guard until 5.40pm.
A ten-hour countdown has now begun, with the vital oxygen supply expected to run out at 7.08am US eastern time (11:08am GMT, 12.08pm UK, 9.08pm Sydney), a US Coastguard spokeswoman has said.
Hopes of finding Titan are now fading, hours after those leading the rescue said they will 'hold on hope until the very end', but admitted that they would at some point need to 'make a tough decision'.
Rescuers including the USCG, British Navy and French and Canadian teams are picking up the pace ahead of as the window of rescue narrows.
They face a battle against time, and with less than twelve hours left of oxygen on the vessel, experts have said those inside will be trying to minimise their breathing to preserve the remaining supplies.
Rescuers are refusing to give up hope, US Coastguard Captain Jamie Frederick insisted on Wednesday, even as people around the world count down the hours until oxygen is expected to run out on the vessel.
'We have to remain optimistic and hopeful when we are in a search and rescue case.
'If we continue to search, potentially we could be at that point... And that's a discussion we will have with the families long before I am going to discuss here publicly.'
According to OceanGate, Titan's operator, the sub has a 96-hour oxygen supply in case of emergencies.
Search and rescue teams are racing against time as they scour an area where noises were detected for the missing Titan sub, with experts saying they are continuing to analyse the sounds which were heard as recently as this morning.
The only possible trace of the vessel which is continuing to be investigated was underwater 'banging' sounds, which were detected yesterday in the search for the missing Titanic submersible.
But, the US Navy experts analysing the sounds said they cannot yet conclude whether they are coming from the stranded vessel, and ROVs were continuing to return negative results from under the sea.
As time runs out, more equipment is expected to arrive as late as Thursday morning, including more remote operated vehicles (ROV) to detect noise as planes continue to scour the ocean from above.
Private groups who had offered their help to the rescue mission as early as Monday were allegedly not authorised to help until Wednesday night, sources also told MailOnline.
Rescuers are now embarking on a last-ditch attempt to find the tiny vessel in a huge expanse of water, with the search area expanded to around 14,000 square miles - twice the size of the state of Connecticut.
Hopes of finding the sub have been pinned on the Victor 6000, which was rushed to the search area overnight and can reach depths of 20,000ft and is being rushed to the search area.
The Victor 6000 ROV may be able to fix a cable onto the sub before it is hauled to the surface by the Flyaway Deep Ocean Salvage System, a specialist winch which in 2021 managed to rescue a helicopter from 19,075ft deep.
The time pressure is compounded by the fact that the underwater vehicles may be able to pinpoint Titan, which was headed for the Titanic wreckage 12,500ft below the surface, but it will take additional specialized tools for the mammoth task of bringing it up.
Hopes of a recovery were raised slightly on Wednesday when a Canadian P-3 aircraft equipped with sonar detected the periodic 'banging' sound.
The Coast Guard chief who's coordinating the search said that sounds were initially heard overnight and more were detected today.
The P-3 is one of several models of aircraft which are also helping the search by scouring the ocean surface and using sonar equipment for signs of activity on the seabed.
A range of military and commercial vessels is also at the site, offering a mixture of search capabilities, communications tools and rescue equipment if Titan is found.
Ultimately, it is down to the submersibles to get eyes on Titan if it remains on the seabed - or trapped within the Titanic's wreckage.
Sean Leet, co-founder of Horizon Maritime Services, the company which owns Titan's mothership Polar Prince, said on Wednesday that he has never seen advanced search 'equipment of that nature move that quickly'.
The family of the missing Titan sub tourists, Pakistani businessman Shahzada Dawood and his 19-year-old son Suleman, earlier gathered above the water where the vessel was last seen as oxygen levels continue to diminish onboard.
A family source in Karachi, Pakistani, where Mr Dawood is from, released a new picture of the father and son to MailOnline and said: 'I can tell you that Mrs Dawood and her daughter are currently in the search area at the moment and will remain there for as long as they can.
'Naturally this is a very tough time for the family and they're not coping at all well with the situation, they are drawing strength from each other and hoping and praying for the best.
'The messages of support they are getting from all around the world are also keeping them positive and they are grateful for everyone's kind thoughts and wishes.'
'It's not clear what it was that made Mr Dawood want to go and visit the Titanic with his son, but he is driven by a passion of exploration, and I understand this was something that had been planned for some time.'
The Dawoods belong to one of Pakistan's most prominent families. Their eponymous firm invests across the country in agriculture, industries and the health sector.
Their family, including Shahzada's wife Christine and daughter Alina, are waiting for news of the pair.
The passengers onboard Titan include British billionaire adventurer Hamish Harding and OceanGate's chief executive and founder Stockton Rush.
In a heartbreaking plea today, one of Mr Harding's close friends Jannicke Mikkelsen warned 'we are losing time'.
The panicked friend told BBC Radio 4's Today programme yesterday: 'I'm nervous. I'm sick to my stomach with nerves. I'm terrified, I'm anxious. I'm not sleeping at the moment. I'm just hoping for good news. Every single second, every single minute feels like hours.'