One of the first priorities for air crash investigators is to locate a plane's two black boxes, which hold vital clues on what caused it to go down.
On Monday, Indonesian authorities found the cockpit voice recorder from a Lion Air Boeing 737 MAX that crashed in October, killing all 189 people onboard.
Investigators have already recovered the jet's flight data recorder, which provided information about the speed, altitude and direction of the plane before the crash.
But the cockpit voice recorder -- which keeps track of conversations and other sounds in the pilots' cabin -- will provide new details for investigators trying to find out why the brand new airliner fell out of the sky.
- What are black boxes? -
Introduced in the late 1950s, black boxes help explain nearly 90 percent of all crashes, according to aviation experts.
All commercial planes are required to have two of them on board -- a flight data recorder and the cockpit voice recorder.
It can take weeks for investigators to analyse the data inside, which is often not released to the public until the information has been thoroughly examined.
- How do they work? -
The recorders are housed in boxes built to survive extreme shocks, fire and lengthy periods underwater.
Despite the name, the two boxes are in fact bright orange, with reflective stripes to make them easier for search teams to spot.
They each weigh seven to 10 kilogrammes (15 to 22 pounds) and can survive as deep as 6,000 metres (almost 20,000 feet) underwater or an hour at 1,100 degrees Celsius (2,012 degrees Fahrenheit).
To make them easier to find, they are fitted with a beacon which can emit a signal for one month.
- Missing at sea -
In January 2004, the black boxes of an Egyptian charter plane that crashed into the Red Sea were found after a two-week search, 1,000 metres below sea level.
And in 2011, after 23 months submerged at a depth of 3,900 metres in the Atlantic Ocean, the intact black boxes of Air France flight AF447 were found, allowing investigators to determine what caused the 2009 crash.
Lion Air flight JT610 came down in water some 30-40 metres deep and both black boxes were found within 10 metres of each other, authorities said.
Long-haul Airbus A350 and A380 passenger jets will soon come equipped with ejectable black boxes that can float, making them easier to find after accidents over water.
