LONDONO MAR DEL PLATA, Argentina - Eight days after the last reported communication from submarine ARA San Juan, the Argentinian navy on Thursday (Nov 23) released news of an explosion near its last known location, effectively ending hopes of survival for 44 crew on board.

The navy lost all contact with the submarine at 7.30am on Nov 15 and the explosion was detected in the same area at 10.31am on that same day.

Asked during a news conference about the fate of the sailors, Argentina's navy spokesman Enrique Balbi said the situation was "critical".

"We are not going to enter into conjecture out of respect for the families," Captain Balbi said, adding that search would continue in the same area.

Concern for the missing submarine and its crew has gripped Argentina and dominated international headlines since it was reported missing on Nov 17, two days after it last made contact.

While the navy did not formally given up hope of finding the crew, relatives have begun referring to their loved ones in the past tense.

If the sailors perished, it would be the deadliest submarine catastrophe since the sinking of the Kursk - a Russian vessel brought down by a misfired weapon in 2000 - and the Argentine military's largest loss of life since the Falklands War of 1982.

Here are seven pressing questions about the submarine tragedy.

1. What could have caused the explosion?

The Argentinian navy said it did not have enough information to say what the cause of the explosion could have been or whether the vessel might have been attacked.

One likely cause is ARA San Juan sinking below its "crush depth", or "collapse depth", at which point its structure will not be able to withstand the water pressure.

The "crush depth" of most submarines is classified, but it is likely to be more than 400m. The ARA San Juan search location straddles the edge of the continental shelf, where ocean depths vary, but reach as deep as 3,000m.

"If a submarine goes below its crush-depth, it would implode, it would just collapse," James H Patton Jr, a retired navy captain, told Associated Press.

"It would sound like a very, very big explosion to any listening device."

In 1963, US nuclear attack submarine Thresher was near its maximum test depth, which, though classified, was around 396m when it radioed that it was having minor problems.

US Navy submarine rescue ship Skylark, which was accompanying the sub, received several fragmentary, garbled messages, followed by silence.

Moments later the chilling sounds of a submarine breaking apart and imploding were heard.

"As they sank, the men aboard would have heard piping and fittings giving way. They would have listened as the ship's hull creaked and groaned, until it finally, deafeningly gave way to massive water pressure. All lives were likely extinguished within a matter of seconds," wrote the National Geographic.

2. Were there similar cases in the past?

It is extremely rare for submarines to sink, Stewart Little, a submariner with the Royal Navy for several decades who now runs the Submarine Rescue Consultancy, told BBC News.

It last happened in 2000, when the Russian guided missile submarine the Kursk suddenly sank to the floor of the Barents Sea after two explosions in its bow during a planned and closely monitored military exercise. It was hours before the Russian government even knew something was amiss.

All 118 men aboard the nuclear-powered submarine died.

After recovering the remains of the dead from the submarine, officials determined that 23 crew members, including the Kursk's commander, had survived the initial accident before suffocating.

3. Did ARA San Juan's recent refits contribute to the disaster?

One of three vessels in Argentina's submarine fleet, German-built ARA San Juan was commissioned in 1985 and most recently refitted in 2014.

During the US$16 million retrofitting to extend its use, the vessel was cut in half and had its engines and batteries replaced.

Experts say that refits can be difficult because they involve integrating systems produced by different manufacturers and even the smallest mistake during the cutting phase of the operation can put the safety of the ship and the crew at risk.

Family members say Argentina had let its military degrade to the point of recklessness. "For 15 years, the navy has been neglected." said Itatí Leguizamón, the wife of Germán Oscar Suárez, a radar operator on the vessel.

"They sent a piece of crap to sail."

Argentina has spent less than several of its neighbours on defence since the end of military rule in 1983.

4. For how long could the crew survive?

Under normal circumstances, the vessel has sufficient fuel, water, oil and oxygen to operate for 90 days without external help, said Captain Balbi, and the vessel could "snorkel" - or raise a tube to the surface - "to charge batteries and draw fresh air for the crew."

If the sub is bobbing adrift on the surface and the hatch is open, it will have an available air supply and enough food for about 30 days, he said.

While submarines of this size and class can stay at sea for around a month, that doesn't mean they have 30 days underwater.

The vessel risked running out of oxygen if submerged for too long.

"It's dependent upon the last time they actually recharged their batteries, how long ago they refreshed the air, what's inside the submarine," William Craig Reed, a former US Navy diver and submariner who writes on the subject, told CNN.

The ARA San Juan would have enough oxygen for its crew to survive underwater for seven days if there was no hull breach.

One of the most important practices is for trapped crew members to slow down their breathing rates in order to conserve oxygen.

Dr Robert Farley, a lecturer at the University of Kentuck, said: "My guess is that they would be cautioned to reduce activity and reduce speaking in order to save oxygen."

They will likely establish routines, making themselves as comfortable as possible while minimising their movements and supporting one another as they await rescue.

The response to these emergency incidents has also been improved at the international level since the Kursk disaster in 2000.

5. Did a battery malfunction cause the explosion?

The Argentinian navy spokesman said on Thursday (Nov 23) there was no evidence that a battery problem reported by the ARA San Juan captain shortly before the submarine went silent was related to the explosion.

On Nov 15, the captain reported a breakdown relating to a "short circuit" in the vessel's batteries caused by an intake of water through the snorkel system (by which it renews oxygen).

But naval commander Gabriel Galeazzi, who heads the naval base in Mar del Plata, said such mechanical problems were not uncommon and rarely posed a risk.

"A warship has a lot of backup systems, to allow it to move from one to another when there is a breakdown," he said earlier this week.

A former submarine commander told AFP a problem with batteries could cause an explosion.

"A severe problem with batteries might generate hydrogen. Hydrogen above a certain percentage is explosive," said the commander, who requested anonymity.

"It explodes by itself. Should they have had an explosion, then what? Everything was lost."

6. Why is it so difficult to find a missing sub?

Submarines, whose role is often to participate in secret surveillance operations, are built to avoid detection.

Dr Farley says that a submarine is very hard to trace if resting on the seabed because under such circumstances it will not be making any "noise".

"Noise, which would otherwise be picked up by what is known as passive sonar, is distorted and (the submarine) looks - to active sonar pings - like the sea bottom," he told BBC News.

A multinational air and sea search is under way with help from seven countries including Brazil, Britain, Chile, the United States and Uruguay. The Argentinian navy's four P3-B maritime patrol aircraft have been grounded and unavailable for deployment, according to Jane's Research.

The US Navy has put more advanced resources into the Atlantic Ocean, including two unmanned underwater vehicles that use side-scan sonar to create images of large areas of the seafloor.

Once the submarine has been located, an underwater rescue vehicle that can take up to six people will be despatched. The vehicle, also known as submarine rescue chamber, can dock with the vessel's escape hatch.

7. How then can the crew send out distress signals?

There are a number of ways that the captain or crew can make their location known if in distress.

These methods include sending signal calls to contacts at naval bases or allied ships, or releasing a device that floats to the surface but remains attached to the submarine.

News earlier this week that noises had been detected raised hopes to find alive the 44 crew members aboard, but the hopes were dashed when the navy later said "the sound footprint could not correspond to a sub's... it may have been a noise from a living thing".

The noises sounded like tools being banged on the hull of a submarine to attract the attention of rescuers, CNN reported, citing an unnamed senior US navy official familiar with the international search effort.

The explosion near the site where ARA San Juan was last located came to light only after analysts from the US government and an international nuclear weapons monitor detected it and told the Argentines.

Captain Balbi on Thursday described it as an "abnormal, singular, short, violent, non-nuclear event".

SOURCES: BBC NEWS, NEW YORK TIMES, CNN, REUTERS, AFP, WASHINGTON POST