The chances of the missing Argentinian submarine being found with any of its 44 crew members alive are fading fast, according to an Australian defence expert.
The ARA San Juan has been missing for five days in rough seas after disappearing 430 kilometres off Patagonia on the country's south-east coast.
Rescue aircraft and vessels are ramping up the search for the sub after a series of apparent attempted distress calls yesterday raised hopes they may be alive, with an electrical problem affecting communications.
But Australian National University professor James Goldrick, former leader of the Australian Defence Force Academy, told news.com.au that it was highly unlikely they had survived after so long.
"Every hour, the likelihood of their being found alive diminishes," said Professor Goldrick, a rear admiral who commanded two ships and led Australia's Border Protection Command.
"I don't hold up much hope of their being found.
"It would be fantastic if it turns out it was just communications, but after this amount of time, I don't believe it will.
"You'd be fearing the worst by now."
'You could have survivors trapped at both ends'
The Argentinian navy was unable to confirm that the seven attempted calls originated from the submarine, after losing contact with the German-built, diesel-electric sub early Wednesday local time. "The communications are so short and the signal so low," spokesman Enrique Balbi said.
Naval expert Fernando Morales told C5N TV there was a feeling of "cautious enthusiasm", since to use a satellite phone, "the submarine had to emerge to a depth that allowed the call."
The navy said there was enough food and air supply on board for the crew to survive for up to two weeks - but if some disaster has happened, that may not count for anything.
But as time wears on, it seems increasingly doubtful the crew is alive.
The ARA San Juan is an old submarine, built in 1984, just after the Falklands War.
Argentina's navy has very little money and a fleet of just three submarines, but it has "been hanging on to its subs by its fingernails", says Prof Goldrick
With the sub hidden deep in the ocean, it is impossible to know what has happened, but the professor speculates that something more than a communications failure has happened to prevent it from surfacing.
Submarines usually have indicator buoys that appear at the surface if a sub goes down, but with the vessel being so old, we don't know what its capabilities were. The fleet was modernised between 2007 and 2014 to extend its use by about 30 years, but Argentina has been plagued by economic problems since the Falklands War.
If a compartment of the submarine has caught fire or flooded, "it gets very tricky to survive", Prof Goldrick added.
"If, let's say, a pipe broke in a major compartment, you have to close that off," he explained. "If there's flooding in the centre of the ship or a fire in the control room, you could have survivors trapped at both ends of the boat."
'Below a certain level, the hull crushes in'
The impact depends on not only how bad the accident was, but also how deep the sub was at the time it happened. "If you're relatively shallow, you're OK, but if it's deep, they're all dead," said Prof Goldrick. "Below a certain level, the hull crushes in."
It would also then be too deep for the crew to use any means of escape such as diving apparatuses because the human body cannot endure the pressure at such depths.
Even if the crew managed to escape the sub at a level shallow enough for them to reach the surface, it's unlikely they would last very long in the cold waters off the San Jorge Gulf.
"It's a rough part of the world," added the professor, whoretired in 2012 after years of sea service. "How long could they survive in the water, even in protective clothing? There's a whole range of things that could have happened."
Relatives of crew members have unfurled a flag at the naval base in Mar del Plata - the sub's intended destination - that reads: "Be strong Argentina, We trust in God, We wait for you."
Claudio Rodriguez, whose brother Hernan is aboard the submarine, said he was hopeful the satellite signals indicated the vessel was still afloat. "They've got to be afloat," he said.
Argentinian President Mauricio Macri said the authorities were doing everything to find the sub as soon as possible. Argentina-born Pope Francis offered a "fervent prayer" for the submarine sailors' safety.
The exact position of the ARA San Juan is still unclear. The TR-1700 class submarine had been returning from a routine mission to Ushuaia, near the southernmost tip of South America, to its base at Mar del Plata, about 400km south of Buenos Aires.
It appears to have taken a detour to gather information on illegal fishing. It is common in the region for foreign boats to exceed their quotas, transferring their haul to other boats so they can continue overfishing the South Atlantic Ocean.
Ten aircraft from countries including Brazil, Britain, Chile, Uruguay and the United States are conducting the search for the lost sub, focusing on an area about 300km in diameter, radiating from the last point of contact.
Rescuers are likely to be equipped with inflatable boats, life rafts, specialist communication equipment, rations, water and medical supplies, all of which can be dropped from parachutes.
California-based Undersea Rescue Command was deploying two underwater craft designed to rescue trapped submarine sailors. A NASA P-3 research aircraft has joined the search and Britain's Royal Navy has just sent Antarctic patrol ship HMS Protector to assist.
A powerful storm has whipped up seven-metre waves, which has made geolocation difficult, officials said. Weather conditions are not expected to improve before Tuesday.