The clock is ticking for rescuers to save the five people trapped in a submarine in the Atlantic Ocean, but there are still some reasons to be hopeful.
Sonor equipment detected what rescuers believe may have been tapping noises from the search area for the missing deep-sea vessel called Titan - which has spurred the search on.
Five tourists, including Britons, paid around £200,000 to OceanGate Expeditions for what should have been an eight-hour trip in Titan to view the Titanic shipwreck.
However, the small submarine-like vessel lost contact around 90 minutes after it submerged on Sunday and crews have been desperately hunting to rescue them since.
Stranded on the vessel are British adventurer Hamish Harding, pilot Stockton Rush, the CEO of OceanGate, two members of a UK-based Pakistani business family and a Titanic expert.
The crew have now likely lost their 96-hour oxygen supply and rescuers have no way of knowing whether the crew are dead or alive.
Despite all this, they have not given up yet and still remain hopeful.
Still an "active search and rescue" mission
Today Rear Admiral John Mauger confirmed the main focus of the US Coast Guard was still search and rescue, implying the crew are alive and not dead.
He added that he "remained hopeful at this point".
John is in charge of the search operation to retrieve the vessel and also revealed that the operation has brought a medical team "out to the site" with a hyperbaric chamber [a machine to help the lungs collect more oxygen].
He told Sky News: "We continue to keep the crew members and the families in our thoughts as we proceed with this search and rescue while we're cognisant of the time and we've factored in a lot of data and information into the search.
"This is still an active search and rescue at this point and we're using the equipment that we have on the bottom right now, the remote-operated vehicles to expand our search capability, and then also to provide rescue capability as well.
"We are using the equipment that we have on the bottom right now, remote-operated vehicles to expand our search capability, and also to provide rescue capability."
He said conditions for the operation were "favourable" - and crews were making the most of the "weather window and the good conditions".
He also confirmed that any decision to stop the rescue effort would be taken after a "thorough consultation" with the families of the passengers on the Titan.
Oxygen may actually last longer
A few experts have publicly said they do not believe the 96-hour oxygen limit is final and think the people on board could have more time.
The co-founder of OceanGate Guillermo Sohnlein claimed today that he "firmly believes" the window available for their rescue is "longer than what most people think."
While Dr Kenneth Ledez, a hyperbaric medicine expert at Memorial University in St John’s, Newfoundland near where the Titan was launched, told the BBC that there is also a slim chance they could hold out longer.
Guillermo said in a statement: "Today will be a critical day in this search and rescue mission, as the sub's life support supplies are starting to run low.
"I'm certain that Stockton and the rest of the crew realised days ago that the best thing they can do to ensure their rescue is to extend the limits of those supplies by relaxing as much as possible.
"I firmly believe that the time window available for their rescue is longer than what most people think.
"I would encourage everyone to remain hopeful for getting the crew back safely."
Dr Ledez said: "It depends on how cold they get and how effective they are at conserving oxygen."
He said shivering would use up a lot of oxygen while wrapping up in a huddle can help to conserve heat.
He said running out of oxygen is a gradual process. "It's not like switching off a light, it's like climbing a mountain - as the temperature gets colder and metabolism falls [it depends] how fast you ascend that mountain," he said.
Dr Ledez said conditions could be different person-to-person, and that although it is a "disturbing conversation", some could survive longer than others.
Frank Owen, a submarine search-and-rescue expert, meanwhile told the Associated Press that the 96-hour oxygen supply figure is a useful "target" for searchers, but is only based on an estimated amount of consumption.
Robot reaches sea floor
A French deep-sea robot that can dive to depths of 6,000 meters has reached the sea floor and rescuers believe it could be the key to finding Titan.
The robot, called the Victor 6000, is fitted with cameras, lights and robotic arms that could, if Titan is found, assist in raising it to the surface.
This robot is the "main hope" for a possible rescue mission, Rob Larter, a maritime expert from the British Antarctic Survey, said at a press conference today.
The Victor can dive up to depths of 6,000m, meaning it can go even deeper than the Titanic wreck itself which lies at about 3,810 metres.
The robot was aboard the French ocean research ship L’Atalante, which was working in the Atlantic, 48 hours away, when the Titan vanished Sunday on its way to the Titanic wreckage, which is at about 3,800 meters (12,467 feet) deep.
"Victor is able to do visual exploration with all the video equipment it has", said Olivier Lefort, who was part of the team that found the wreck of the Titanic in 1985.
