The Argentine military submarine ARA San Juan and crew are seen leaving the port of Buenos Aires, Argentina June 2, 2014. Picture taken on June 2, 2014. Armada Argentina/Handout via REUTERS

Eleven days after Argentina’s missing San Juan submarine went silent following an explosion, a 14-nation search has failed to find the vessel at the bottom of the Atlantic Ocean.

But not for want of trying.

For more than a week, aircraft from Argentina, Britain and the United States have crisscrossed the South Atlantic.

A Russian Antonov transport plane has arrived with an underwater robot that can scour the ocean at a depth of 1,000 meters (3,300 feet), adding to the arsenal of sophisticated international recovery tools.

Russia also sent an oceanographic research ship to the search zone, and the US Navy provided an underwater rescue capsule.

Even though Argentina’s navy has yet to declare the 44 crewmembers of the ARA San Juan dead, many relatives of the crew have lost hope.

On Thursday, the navy revealed there had been an explosion aboard the submarine, which experts said was likely catastrophic and linked to a battery problem.

“There is no precedent in history for a deployment of this extent,” naval engineer Horacio Tettamanti said of the recovery effort.

“The United States and Russia are the most developed in this field, a legacy of the Cold War,” added Tettamanti, one of Argentina’s leading experts in the field.

Confirmation of the explosion has led to a more localized search area around a zone 400 kilometers off the Argentinian coast, after searchers initially scoured a 500,000 square kilometer (193,000 square miles) area nearly the size of France.

But in this region north of the Falkland Islands — known in Argentina as the Malvinas — depths at the edge of the Argentine shelf can plummet.

From Argentina’s military base at Bahia Blanca two US Navy P-8A Poseidon aircraft are using their radars and scanners in the sub hunt.

An AFP journalist aboard saw them drop buoys equipped with sensors to try to detect the vessel.

US personnel focus on their monitor screens, looking for any clues to the sub’s location on a mission that continues 24 hours a day using rotating crews.

At the port of Comodoro Rivadavia, a ship carrying the submarine rescue vehicle cast off and headed toward the search zone.

The vehicle could descend to the sea floor to recover the crew members once the San Juan is located — and it will be, said Tettamanti.

“The search will continue until they find it. With the technical deployment that is there I am convinced that it will turn up rapidly, in coming days,” he said.

But military expert Rosendo Fraga cautioned: “The search is going to take time.”