While search crews scramble to find wreckage from flight MH370, two virtually indestructible devices are likely sitting on the ocean floor emitting "pings".
The plane's flight data recorder and cockpit recorder can hold the keys to unlocking the cause of a crash. The boxes - in reality coloured orange so they can be seen - can withstand an hour in a blazing inferno, the pressure of 6,000 metres of water, or a high impact that would obliterate most other plane parts.
But once they hit water, the battery-powered signal they send out can transmit for only a month. And search teams have to be within about four kilometres to pick up the signals.
Watch: What is a black box?
If search crews can confirm that satellite images of floating debris in the southern Indian Ocean is from the Boeing 777, they will calculate where the bulk of the plane may have come to rest on the seabed and head there to listen for the pings.
What does the black box record?
The data recorder logs performance and other metrics, including speed, altitude and direction. It can give investigators a cache of information of the 25 hours prior to a crash. The voice recorder captures two hours of sound from microphones in the cockpit. It runs on a loop, so audio from the critical moments during which the plane diverted west from its Malaysia-China route - about seven hours before it is believed to have crashed - would have been erased.
How long will it transmit a signal?
Each recorder has a beacon bolted to the box's exterior, which once activated by water emits a chirp every second that requires special equipment to detect. A beacon's battery is designed to last 30 days. Experts say another five days' useful pinging is possible as the batteries fade. The Malaysia Airlines flight is believed to have crashed on March 8. Time is running out.
Why only a month?
After the 2009 crash of an Air France flight in the Atlantic Ocean, search teams did not find the black boxes for nearly two years. Aviation regulators began a push to extend battery life, and the European Aviation Safety Agency will require a 90-day pinger starting next year. Several years later, a second regulatory change kicks in. A much larger pinger must be added to the structure of the airframe that would give searchers a second signal to track, detectable up to 13 kilometers away.