New data analysis has led Malaysia Airlines to conclude that flight MH370 came down in the sea - but the data alone cannot reveal why it did so.
Key to finding out what caused the Beijing-bound plane to end up in the Indian Ocean are the cockpit voice recorder and the flight data log, both located inside its black box.
Australian and Chinese authorities located several objects that might be debris in the southern Indian Ocean in the 24 hours before Malaysian Prime Minister Najib Razak told the world that all those on board had been lost.
But the exact site of the crash has not yet been identified, and there is now a race against time to find the box before its locator beacon runs out of power.
Experts say that if it is not recovered, the cause of the crash may never be known.
Colonel Paul Edwards MBE, a former Army Aviation chief of staff, told Sky News that strong currents and high waves would make it difficult to find the precise point of impact.
"It's a huge challenge. First you have to locate the debris from the accident in currents which are swirling all over the place and in high seas," he said.
Wreckage from the plane may have drifted a long way from the original site, he added.
The black box could be attached to a large piece of debris from the plane, or it may have become detached and sunk to the sea floor.
Finding it as quickly as possible is crucial, as its locator beacon - which sends signals telling searchers where it is - may lose power after about 30 days.
The United States has sent a "Towed Pinger Locator" to the region, which is pulled behind a vessel at slow speeds.
It has a highly sensitive listening capability and can detect "pings" from a black box emitted at a depth of 20,000ft (6,000m).
The sea in the southern Indian Ocean is up to 13,000ft deep - well inside the range at which depth the Boeing 777's black box recorder is designed to work.
Philip Baum, editor-in-chief at Aviation Security International, said that while the ocean was deep, the sea floor was relatively flat, which should help efforts to find it.
"We need to find out what brought about the incident in the first place and the black box is absolutely critical," he told Sky News.
"You could say that unless you find a significant piece of wreckage that indicates an explosion brought the plane down, then the black box is going to be the only way to determine what occurred."