Black boxes - which are actually orange - are a group of data collection devices mounted in the tail of an aircraft.
Under internationally agreed regulations, commercial aircraft must carry the equipment to record the performance and the condition of the aircraft in flight.
The recorders are housed in immensely strong materials, such as titanium, and insulated to withstand a crash impact many times the force of gravity, temperatures of more than 1,000 deg C for up to 30 minutes and the immense pressure of lying on the seabed.
One manufacturing test for data recorders involves firing them from a cannon into a wall to simulate an aircraft suffering a catastrophic crash landing while traveling at hundreds of miles an hour.
The recording material is itself insulated against accidental deletion and the corrosive effects of sea water.
Modern black boxes record up to 300 factors of flight including:
Airspeed and altitude
Heading and vertical acceleration
The safety precautions are designed to ensure, theoretically, that accident investigators will be able to recover the recorders, compile a full picture of an aircraft's last moments from the recordings and then accurately explain what went wrong.