As the desperate, multinational search for a missing Argentine submarine continues, the countrys navy said on Thursday that a sound consistent with an explosion had been detected near the site where the vessel disappeared last week with 44 crew members on board.

Captain Enrique Balbi, a navy spokesman, said an abnormal, singular, short, violent, non-nuclear event had been detected in the South Atlantic ocean on the morning of Nov. 15. That was around the same time the ARA San Juan sent out its last signal, reported Reuters.

The search for the missing vessel, which has enough air to last only seven to 10 days if fully immersed, has been intensifying as several countries participate in a massive search covering some 187,000 square miles.

Heres what we know about the search so far:

When did the San Juan last make contact?

The last communication between the diesel-electric powered submarine and onshore authorities was at 7.30 a.m. on Nov. 15. The vessel had been heading from the southern port of Ushuaia to a naval base in Mar del Plata, about 260 miles south of Buenos Aires. It had been scheduled to arrive at its destination on Sunday.

Shortly before disappearing, the San Juans captain reported a battery failure due to a short circuit on board. He later said the issue had been fixed.

According to the Argentine navy, such failures are considered routine and the vessels crew had been reported safe at the time , per CNN . Still, the captain was reportedly ordered to change course and take a more direct route to Mar del Plata.

On Saturday, the Argentine Defense Ministry said it had detected seven communication attempts from the San Juan. That, however, turned out to be untrue.

What could have happened?

The submarines apparent failure to use any distress signals or mechanisms has troubled experts because it suggests the submarine could have been quickly and devastatingly incapacitated by a cataclysmic event onboard , noted the Los Angeles Times.

William Craig Reed, a former U.S. Navy diver and submarine expert, told CNN that the submarine could have suffered some sort of catastrophic failure or it could be something minor that has caused them to either be hung up somewhere or they are on the bottom.

The Argentine navy said on Thursday that there was no sign the submarine had been attacked .

How is the search effort going?

Thousands of military personnel, more than a dozen aircraft and 15 vessels from countries including Brazil, Britain, Chile, Colombia, France, Germany, Peru, the United States and Uruguay are searching for the vessel.

Finding a submarine, which is difficult to detect by design, is significantly more challenging than locating a surface vessel, experts say.

The search has also be hampered by poor weather, with storms causing strong winds and waves of up to 20 feet in the search area.

How long can the submarine last at sea?

Balbi, the Argentine navy spokesman, said the submarine has enough food, water, fuel and oxygen to operate for 90 days without external help. However, that only applies if the vessel can raise a snorkel to the surface to charge batteries and draw fresh air for the crew.

If the submarine has been fully submerged and unable to surface, its oxygen supply may only last about seven days.

Wednesday marked the seventh day since the vessel disappeared.

I feel like Im waiting for a corpse , Helen Alfaro, the sister of radar officer Cristian Ibaez, told reporters last week.