BUENOS AIRES, Argentina — The Argentine submarine ARA San Juan went missing in the South Atlantic last week with 44 crew members aboard. Here’s a look at the submarine and the round-the-clock international maritime search.


THE VESSEL

The German-built diesel-electric TR-1700 class submarine was commissioned in 1985 and was most recently refit in 2014.

The retrofitting cost about $12 million and took more than 500,000 work hours. The boat was cut in half and had its engines and batteries replaced.

Refits can be difficult because they involve integrating systems produced by different manufacturers, said Rockford Weitz, director of the Fletcher School’s maritime studies program at Tufts University.

“The cost of even the smallest mistake during this cutting phase of the operation is enormous – threatening the life and safety of the ship’s crew,” Weitz said.


LOST AT SEA

The Argentine navy says it lost contact with the submarine on Nov. 15. It had sailed from the extreme southern port of Ushuaia on Nov. 8 after a training exercise and was heading for its base at Mar del Plata, about 250 miles (400 kilometers) southeast of Buenos Aires.

Most submarines can deploy a location beacon to the surface that can emit emergency signals via satellite, but there is no sign the San Juan did so.

The sub carried enough food, oxygen and fuel for the crew to survive about 90 days on the sea’s surface, but the navy said it had only enough oxygen to last seven days if submerged. Other experts, however, said that if the sub sank but was still structurally intact, the crew could have seven to 10 days of oxygen.

The amount of oxygen would depend on when the San Juan last resurfaced to recharge its batteries and other factors. “But it is clear that time for a successful rescue operation is very, very limited,” Weitz said.


POSSIBLE PROBLEMS

The submarine’s captain reported a battery failure and the vessel was on its way to the navy base in Mar del Plata when it went missing. Authorities have no specific details of the problem.

Argentine naval protocol says that when a sub loses communications, it should surface. But navy spokesman Enrique Balbi said the crew might have remained submerged to protect the sub from stormy weather that has caused waves of more than 20 feet (6 meters).

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AN EXPLOSION?

The navy announced Thursday that a sound detected near the site where the sub vanished apparently came from an explosion. Balbi said evidence showed “an anomalous event that was singular, short, violent and non-nuclear that was consistent with an explosion.”

He said it’s not yet clear that the sound came from the vessel. But the ominous news led relatives of the missing sailors to burst into tears.


INTERNATIONAL HELP

More than a dozen vessels and aircraft are searching off the coast of the Patagonia region in southern Argentina. The sub’s last known position has been combed fully, and the search area has been expanding. The effort has been hindered by the bad weather, though forecasters say conditions should improve in the coming days.

Britain has sent a polar exploration vessel, the HMS Protector, and the U.S. Navy deployed its Undersea Rescue Command, which includes remotely operated vehicle and vessels capable of rescuing people from bottomed submarines.


CREW AND FAMILIES

The San Juan had a crew of 44, which included Eliana Krawczyk, Argentina’s first female submarine officer.

Worried relatives of the missing sailors have gathered at the Mar Del Plata Navy Base to receive psychological counseling and anxiously wait for news about their loved ones.

“We can make up a thousand movies with happy and sad endings, but the reality is that the days pass by and not knowing anything kills you,” said Carlos Mendoza, the brother of submarine officer Fernando Ariel Mendoza. “Every minute is oxygen that’s worth gold.”


Associated Press writer Almudena Calatrava in Buenos Aires and AP video journalist Paul Byrne in Mar del Plata, Argentina, contributed to this report.