The missing submarine could have just one day of air supply remaining if it is still under the surface as the international search reaches a critical point.

The ARA San Juan lost contact with land last Wednesday morning local time after leaving the port of Ushuaia in the country's hostile south to sail around the tip of the Patagonia region.

An Argentinian navy official said the sub had reported a battery failure and was returning to base in Mar del Plata when it vanished around 430km off the south-east coast.

The ARA San Juan diesel-electric submarine has been missing since last week.
The ARA San Juan diesel-electric submarine has been missing since last week.

Relatives of the 44 missing crew members are now gathered at the base as they anxiously wait for news, with hope fading fast.

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Argentinian navy spokesman Enrique Balbi said on Monday that the sub could run out of oxygen in just two days if it was beneath the surface, but would have enough air for a month if it was bobbing on the surface.

"In the worst-case scenario, in the critical phase, where it could not come to the surface by its own means or renew its air and oxygen, we'd be in the sixth day of oxygen," he said overnight.

'I WOULD BE SITTING ON THE BOTTOM AND WAITING'

Unlike modern nuclear subs, the 34-year-old, diesel-powered vessel must come up for air every seven days to replenish the cabin's oxygen supplies and allow its four engines to charge its batteries.

The submarine has emergency canisters that would provide an extra few days of oxygen and remove toxic carbon dioxide, but there is a possibility that it may not be functioning because the vessel is out of power.

Captain Gabriel Galeazzi talks to journalists about the missing submarine at the Navel base in Mar del Plata, Argentina. Photo / AP
Captain Gabriel Galeazzi talks to journalists about the missing submarine at the Navel base in Mar del Plata, Argentina. Photo / AP

Frank Owen, from the Submarine Institute of Australia, told news.com.au that the crew could well be waiting underwater for their chance to surface.

"We know it's been really rough, so even though the Argentinian navy protocol is to surface, it makes no sense with 6-8m waves to be sitting on the surface," he said.

"If I was in that situation, I would be sitting on the bottom and waiting, doing everything I could to slow down my metabolism and eke out the life support mechanisms, reduce oxygen use and CO2 creation."

It is only now the weather has subsided somewhat that rescue teams will be able to comb the area properly for the submarine.

Juan Carlos Mendoza, father of crew member Fernando Mendoza of the missing submarine ARA San Juan, waits for news. Photo / AP
Juan Carlos Mendoza, father of crew member Fernando Mendoza of the missing submarine ARA San Juan, waits for news. Photo / AP

We are now entering a crucial couple of days for the sub to make an appearance, six days after it vanished. The rough seas and stormy weather that have hindered the search cleared yesterday, helping rescue teams cover a wider area, and they will now be able to drop small explosives to help the sub find them. But lingering winds and cloud mean conditions are still not ideal to find the vessel.

Owen added that while "nominally" the crew had seven days of air, "you can never be sure how well these systems work." He said that if the sub released a flotation device to mark its position at the surface, it would most likely have been ripped off by the stormy sea.

"Time is running out, there's no question of that," he said. "Hope is fading."

'PAIN, HELPLESSNESS, AT TIMES HOPE'

Authorities suspect the 66m-long vessel could be more than 180m under the surface as supplies dwindle. It is unlikely to be able to withstand depths of more than 365m without imploding from the pressure.

The Argentinian navy believes that if the sub sank, it did so on the continental shelf, which is shallow enough for the hull to not have crushed inward.

A ribbon with the colors of the Argentine flag and a rosary hang from barbed wire outside a Navy base in Mar del Plata. Photo / AP
A ribbon with the colors of the Argentine flag and a rosary hang from barbed wire outside a Navy base in Mar del Plata. Photo / AP

A United States aircraft assisting with the desperate search spotted white flares overnight, but Enrique Balbi said they were unlikely to be from the submarine, which has red and green flares.

The navy spokesman added that a life raft found early yesterday did not belong to the submarine and most probably fell off another vessel.

"For now, based on the colour, they don't belong to the submarine," Balbi said. "It's quite common that ships pass by that area and also common that with the waves and the rocking, they can lose a raft."

Friends and family of the crew have had their hopes dashed repeatedly thanks to a series of red herrings. Searchers were at first optimistic that seven failed satellite calls could be from the sub, but experts concluded they were not.

There was also excitement when sounds were detected deep in the South Atlantic that officials thought could be tools being banged against the hull of a vessel — before it was determined they were most likely from a "biological" source.

"A little glimmer had started to shine and then it went out," Maria Morales, mother of one of the submariners, told reporters as she arrived at the Mar del Plata base yesterday.

"There's a mix of feelings: pain, helplessness, at times hope."

'IT MAY ALREADY BE TOO LATE'

Submarines are built to be hard to find, and if the San Juan is resting on the sea bed, it would not be visible to sonar pings.

The initial search zone was 300km in diameter with a depth of up to 350m, but it could be expanded to seven times the area, officials said.

An Argentine national flag with a drawing of a submarine hangs from the fence of the naval base in Mar del Plata, Argentina. Photo / AP
An Argentine national flag with a drawing of a submarine hangs from the fence of the naval base in Mar del Plata, Argentina. Photo / AP

Ships and aircraft from nine countries including Brazil, Britain, Chile, Colombia, France, Germany, Peru and Uruguay are assisting the resource-stretched Argentinian navy.

The US Navy has sent its Undersea Rescue Command to Argentina to support the search for the submarine. The command includes a remotely operated vehicle and vessels that can dive 1524m and are capable of rescuing people from sunken submarines such as the San Juan.

"There is a highly skilled US team down there now, so if anyone can find and rescue them, they can," former US navy diver and submariner William Reed told Dow Jones. "But it may already be too late."

The most recent sub recovery by the US navy was of the USS Scorpion in 1968. The operation took six months and 99 crew members died.

"The bottom of the ocean is very confusing, so it's like a needle in a haystack," said Jerry Hendrix, senior fellow at Washington think-tank the Center for a New American Security.

Fernando Morales, a navy investigator, said he hadn't seen "signs that the worst has happened" and the crew may have chosen to remain submerged to avoid the storms.

There are grave fears over what horror could have taken place under the sea.

A woman stands before a fence blanketed with national flags, in support of the crew members of the lost submarine at the naval base in Mar del Plata, Argentina. Photo / AP
A woman stands before a fence blanketed with national flags, in support of the crew members of the lost submarine at the naval base in Mar del Plata, Argentina. Photo / AP

In 2000, Russian nuclear sub the Kursk caught fire and exploded underwater, killing all 118 on board — some instantly, others over days. A Chinese sub accident in 2003 killed 70 crew members, who apparently suffocated after what Beijing termed "mechanical problems".

The submarine was scheduled to arrive at the base in Mar del Plata, about 400km south-east of Buenos Aires, on Monday. It is believed to have enough food and water for a month, so if it has surfaced, the crew could survive for some time.

But rear admiral James Goldrick, a former commander turned naval analyst at the Australian National University told news.com.au he didn't "hold up much hope" the crew would be found alive.

"Every hour, the likelihood of their being found alive diminishes," he said. "You'd be fearing the worst by now."