Argentina's missing submarine: Key questions on disappearance of ARA San Juan

The submarine ARA San Juan has been missing since last Wednesday. Here, Alistair Bunkall tries to unravel some of the mystery.

What could have happened to missing sub?

Short of guessing, nobody knows. Its last known contact was 267 miles off coast in the Gulf of San Jorge last Wednesday. It could have suffered a small fire which has knocked out its communications, a leak that has hampered its ability to resurface or something more catastrophic might have occurred.

:: Mystery deepens over missing sub amid distress signals

What do we know of the location it was sailing in?

Worryingly its last known contact was close to a continental shelf. If it is the right side of that, it might have come to rest on the bottom around 600 metres below water. Although deep, this is just within the safe limits for this submarine. If it sunk the other side of the cliff edge then it would probably have gone beyond its crush depth and its hull would have buckled under the pressure.

Has the crew made contact?

Seven aborted satellites calls were picked up at the weekend giving hope of life on board. They each lasted between 4-36 seconds. The Argentinean Navy has now said those calls didn't come from the submarine.

How long can the crew survive?

Unlike the UK and US submarines which are nuclear powered, the ARA San Juan is diesel electric, meaning that it has a finite supply of fuel, food and oxygen. The best case scenario is that it is on the surface somewhere meaning that low oxygen supplies aren't a factor. If it is submerged, and intact, it might have had as much as 10 days of air - even so, that means time is now running dangerously short. A fire on-board, if that had happened, would have consumed oxygen too.

This is now a multi-national search - who is involved?

Argentina, as you would expect, has sent 12 ships to the search operations.

Neighbouring countries Brazil, Chile and Uruguay have also donated resources to the hunt. Ten aircraft have also been drafted in to help. The Americans have taken over command of the operations though, using A P8 Poseidon Maritime Patrol Aircraft and NASA's P3 Orion. They are looking around the last known location for the sub and along the route it planned to take.

What contribution has Britain made to the search?

:: HMS Protector helps hunt for missing Argentinian submarine ARA San Juan

HMS Protector, the Navy's Ice Patrol ship, is now in the search area. She has a mull-beam echo sounder on her hull that can scan the depths for sign of life. HMS Clyde is returning from South Georgia - she will take a few days sailing to reach the area. A C130 Hercules is on standby in the Falkland Islands if needed.

The British have also sent something called the SPAG - what's that?

The Submarine Parachute Assistance Group is a little known but highly skilled unit. The group - fewer than 10 personnel - can deploy at six hours' notice.

If required they can parachute into the seas to rescue stranded submariners. The SPAG is made up of underwater medics, engineers and specialist escape technicians. Equipment pods dropped with them carry inflatable boats, food, water and other supplies. They can function as a first-response for more than 24 hours whilst other ships make their way to the scene.