[Bagor]

http://news.bbc.co.uk/2/hi/americas/8082025.stm

A French government minister has said the black box flight recorders are believed to be at a depth of between 3,660m (12,000 ft) and 3,700m. At this depth, pressure is immense and there is no daylight. Any search to locate the flight recorders and any plane wreckage will involve a number of technologies. Below we outline the main methods of salvaging wrecks, from divers to the latest deep sea exploratory vehicles.

...Pinger Locator System: This is a specialised listening device that is towed at a depth of up to 4,000m by a ship. The device listens out for the sound of the pinger which is part of the flight recorder. It is activated on contact with sea water and every commercial aircraft carries one. It will emit a signal for up to 30 days.
Side Scan Sonar: Once the pinger is located, a more detailed survey of the area can be carried out. The SSS is a cigar-shaped tube that is towed by a ship to map the seabed in a designated pattern. "You attach it to a cable and you mow the lawn," says Tim Janaitis, Director of Business Development at Phoenix International, a specialist marine salvage company. Mr Janaitis said he believed only the US Navy had the sophisticated pinger locator system to operate at the depth the Air France aeroplane is believed to be.
Mini submarine: France has dispatched a boat with a mini-submarine, the Nautile, aboard. This can operate at a depth of 6,000m, but it was not expected to reach the zone until early next week.