The remains of the Air France jet which went missing over the Atlantic last week are thought to be lying about 6,000m deep, making the job of finding them extremely difficult.
Searchers are concentrating on finding the black box flight recorders, which are likely to provide the best clue as to what caused the crash.
Flight recorders are fitted with emergency locator beacons or pingers which should have begun emitting a signal on contact with the water. Once these have been located, a submarine can be sent down to investigate.
Scuba diver: US Navy divers were used to retrieve bodies and light debris from TWA flight 800 which crashed into the Atlantic off New York in 1996. The plane was discovered at a depth of 40m, within the maximum operating depth for divers which is typically 50m.
Towed pinger locators: These are specialist listening devices that can be towed at a depth of up to 6,000m by a tug. The device listens out for the "sound" of the emergency beacon or pinger, which is attached to the flight recorders. It is activated on contact with sea water and every commercial aircraft carries one. It will emit a signal for up to 30 days.
France has dispatched a boat with a mini-submarine, the Nautile, aboard. This is expected to arrive at the crash site on Wednesday. The Nautile is 8m long and 2.7m wide and can operate at a depth of 6,000m. It can carry a crew of three, and has oxygen to last up to eight hours underwater. As well as cameras and powerful lights, the sub is equipped with two grappling arms and it can also carry a tiny robot probe, which can be used to explore inaccessible and dangerous wreckage. The Nautile is best known for diving to the sunken liner, Titanic.
Remote Operated Vehicle: These are highly sophisticated yet robust underwater vehicles that can operate at depths up to 6,000m. They have video and powerful lights to illuminate the gloomy deep waters that they operate in. They can also have mechanical arms attached that allow the ROV to pick up bits of debris or attach straps to enable a ship's winch to lift the item to the surface. Phoenix International says it has raised a portion of an Israeli submarine weighing 3,600kg from a depth of 3,000m. The US navy has raised an entire helicopter from a depth of 6,000m.
Other devices which can be used in underwater searches include:
Bathymetric survey: This is a sonar device placed beneath a ship that would sail in a designated pattern over an area to map the seabed. It "looks" straight down to produce a 3-D map of the seabed but typically operates up to a depth of 1,000m
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. It can operate at depths up to 4,000m.