KUALA LUMPUR: A year on, there remains no evidence to indicate what caused Malaysia Airlines Flight MH370 to vanish or where it ended up, despite the most expensive search operation in history.
Here are answers to some key questions still swirling around MH370:
Q: What is the status of the search?
A. Vessels scanning the sea floor for wreckage using sophisticated sonar have covered about 40 percent of a "priority search area" in the remote southern Indian Ocean spanning 60,000 square kilometres (23,000 square miles).
Nothing has been found yet apart from several shipping containers in the Australian-led operation, which is due to be completed in May. Australian Prime Minister Tony Abbott said on the one-year anniversary of the disappearance that the search could be extended to another 60,000 square kilometre zone if nothing turns up by then, but offered no further details.
The stormy southern hemisphere winter is expected to begin affecting any future operations within months.
Q: What happens if wreckage is spotted?
A: An immensely challenging recovery phase would begin in pitch-black depths of up to 4,000 metres (13,100 feet) below the surface, potentially hampered by volcano clusters, undersea mountains, ridges and valleys.
Search coordinators, however, can draw on lessons learnt during the quest for the data recorders from the 2009 crash of Air France Flight 447 in the Atlantic Ocean.
Its black boxes were located after a difficult two-year search using submersible drones and other means. A remotely operated vehicle was eventually sent down to pluck them from the seabed nearly 4,000 metres below.
Q: Are they looking in the right place?
A: That remains unclear, although search coordinators insist they are.
The crash zone was determined through analysis of signals from MH370 that were detected by a satellite, which indicated the plane's last known location as being along one of two arcs: one stretching north into Central Asia, one south into the Indian Ocean.
The northern corridor has been discounted in the belief the plane would have been spotted and most experts concur that, while the satellite data analysis is imprecise, the Indian Ocean is the best bet.
But the failure to find anything has sustained nagging doubts - particularly among the families of passengers - about whether the search is on the right track.
Q: What are the main theories today on what happened?
A: Speculation remains focused primarily on a possible mechanical or structural failure, a hijacking or terror plot, or rogue pilot action, but still nothing has emerged to substantiate any of these scenarios.
The lack of solid information has sustained a cottage industry of conspiracy theories, with books, documentaries and a thriving online debate positing a range of possibilities.
These include suggestions that the plane was commandeered to be used as a "flying bomb" headed for US military installations on the Diego Garcia atoll, and was shot down by the Americans. The United States has dismissed this.
A few months after MH370 vanished, former Malaysian prime minister Mahathir Mohamad was among those subscribing to online speculation that the CIA took remote control of the US-made plane after it was commandeered by terrorists.
He added that it was possible "the plane is somewhere, maybe without (Malaysia Airlines) markings".
Writing in New York magazine last month, US aviation expert Jeff Wise sparked an online debate by suggesting MH370 was commandeered to a Russian facility in Kazakhstan, possibly an effort by President Vladimir Putin to intimidate the West during the Ukraine crisis, or to gain access to a certain passenger or item in the hold.
"There's no way to know. That's the thing about MH370 theory-making: It's hard to come up with a plausible motive for an act that has no apparent beneficiaries," he wrote.
Q: Do we know everything the authorities know?
A: Malaysia's government and the airline have continually insisted they are hiding nothing.
But angry next of kin have railed at contradictory early statements by authorities and the carrier, and accused them of being slow to share facts or of divulging only partial information.
Tim Clark, the chief executive of Gulf airline Emirates, said in media interviews last year he also doubted there had been full disclosure.
"I do not believe that the information held by some is on the table," he was quoted as saying.
An independent team of investigators tasked with probing the mystery released an interim report on its findings on Sunday on the one-year anniversary in Kuala Lumpur, with authorities stressing its "interim" status due to the lack of hard evidence of what happened to the plane.
Q: How rare are cases like this?
A: According to the Netherlands-based Aviation Safety Network, which tracks air incidents, there has been only one other recorded instance in which a plane carrying more than 100 people has disappeared without a trace.
That was in 1962, when a turbo-prop operated by US-based Flying Tiger Line and chartered by the US military disappeared en route from Guam to the Philippines with 107 people aboard. Its fate remains unknown.
Q: What will be MH370's aviation legacy?
A: Unless MH370's black box or other telling wreckage is recovered, the aviation industry will be unable to determine what went wrong and consider implementing appropriate safeguards.
But MH370 has also spurred efforts to reduce the chance of planes disappearing.
A global aviation summit in Montreal last month backed plans to require real-time tracking of any airliner in distress starting in 2016.
Last Sunday, Australia also said it was conducting trials, with Malaysia and Indonesia, of a system that increases the frequency with which planes are tracked over remote oceans.