There are few known facts about the disappearance of Malaysia Airlines flight 370, sparking countless theories about its fate and those of the 239 passengers and crew on board.
Here are some of those theories about one of the world's most baffling aviation mysteries:
1. SOURCE: Chris Goodfellow, who describes himself as an experienced pilot.
THE THEORY: Goodfellow has posted on Google Plus that he believed the pilot on MH370 may have been heading to the Malaysian resort island of Langkawi to land after the transponders were knocked out by a fire on board.
"The left turn is the key here. This was a very experienced senior Captain with 18,000 hours,'' Goodfellow wrote.
"Maybe some of the younger pilots interviewed on CNN didn't pick up on this left turn. We old pilots were always drilled to always know the closest airport of safe harbour while in cruise.
"Instinctively when I saw that left turn with a direct heading I knew he was heading for an airport. Actually he was taking a direct route to Palau Langkawi a 13,000 foot strip with an approach over water at night with no obstacles. He did not turn back to Kuala Lampur because he knew he had 8000 foot ridges to cross. He knew the terrain was friendlier towards Langkawi and also a shorter distance.
"Take a look on Google Earth at this airport. This pilot did all the right things. He was confronted by some major event onboard that made him make that immediate turn back to the closest safe airport."
Goodfellow continued: "What I think happened is that they were overcome by smoke and the plane just continued on the heading probably on George (autopilot) until either fuel exhaustion or fire destroyed the control surfaces and it crashed. I said four days ago you will find it along that route - looking elsewhere was pointless."
2. SOURCE: Desmond Ross, an Australian commercial pilot and aviation security expert.
THE THEORY: Captain Ross offers a similar theory to Goodfellow's. He said the aircraft might have depressurised for some reason, possibly due to an explosion causing a hole in the fuselage.
"The pilots quickly recognise the need to descend,'' said Captain Ross, who conducted a security review of Kuala Lumpur's international airport in 2005.
''One of them starts to reprogram the flight management system and sets a low attitude and starts to reset the heading to turn back to Kuala Lumpur ... however he passes out before completing the entries into the computer for the new heading.
''The aircraft climbs out of control due to the explosion on board and then stalls at somewhere between the cruising height and 45,000 feet.
''It falls out of control to the height the pilot had set into the flight management system but does not complete the turn back to Kuala Lumpur because the pilot had only partly entered the numbers ... it flies off on an unknown path.''
Captain Ross stressed that he has no direct knowledge of the investigation.
3. SOURCE: Keith Ledgerwood, who described himself as a hobby pilot and aviation enthusiast from the US.
THE THEORY: This theory was based on the premise that someone hijacked the plane.
Ledgerwood said MH370 could have switched off its radar, then "shadowed" another plane - Singapore Airlines flight number 68 en route from Singapore to Barcelona - before landing north of India or Afghanistan.
"It is my belief that MH370 likely flew in the shadow of SIA68 through India and Afghanistan airspace," wrote Ledgerwood on Tumblr.
"As MH370 was flying 'dark' without transponder/ADS-B output, SIA68 would have had no knowledge that MH370 was anywhere around and as it entered Indian airspace, it would have shown up as one single blip on the radar with only the transponder information of SIA68 lighting up ATC and military radar screens."
4. SOURCE: Described in The Independent newspaper.
THE THEORY: The plane may have been hijacked and flown to a Taliban base.
The Independent has reported that full diplomatic permissions were being sought in order to rule out the theory that the plane could have flown under the radar to areas of Afghanistan and Pakistan that were not under government control.
Large areas of the southern half of Afghanistan were ruled by the Afghan Taliban, while some areas of north-west Pakistan, adjacent to or near to the Afghan border, were controlled by the Pakistani Taliban.
Pakistani civil aviation officials have said that they had checked their radar recordings and found no sign of the missing jet.
And a commander with the Pakistani Taliban said the fragmented group could only dream about such an operation.
"We wish we had an opportunity to hijack such a plane," he told Reuters by telephone.
5. SOURCE: Investigators
THE THEORY: Authorities were investigating whether pilot or passenger suicide could explain the plane's disappearance.
There was a historical precedent for this.
In 1997, SilkAir flight 185 crashed in Sumatra, killing all 97 people on board when the pilot flew the aircraft into a river.
Pilot suicide was also suspected in the crash of an EgyptAir flight in 1999, although that was disputed.
Such an option may explain the lack of debris if flight MH370 went down into the sea. The plane would not have disintegrated before impact, and would have sunk quickly.
6. SOURCE: An email to journalists claiming to be from the Chinese Martyrs' Brigade.
THE THEORY: A shadowy group called the Chinese Martyrs' Brigade claimed responsibility for the disappearance of Malaysia Airlines flight MH370 in an email to journalists in China.
The encrypted email read: ''You kill one of our clan, we will kill 100 of you as pay back.''
But the message provided no details of what brought the flight down.
Malaysia's acting Transport Minister Hishammuddin Hussein has since said he doubted the claim's legitimacy.
"There is no sound or credible grounds to justify their claims,'' he said, according to Malaysian news reports.
7. SOURCE: Malaysian police were said to be investigating this theory.
THE THEORY: An explosive may have been hidden in the plane's huge cargo of exotic fruit.
Malaysian officials have said that the only cargo on MH370 was a load of mangosteens, a fruit popular in south-east Asia.
Malaysia Airlines CEO Ahmad Jauhari Yahya said that "three to four tonnes" of mangosteens were on the aircraft.
He said the cargo had been screened as a matter of routine before being loaded on to the plane, but authorities were said to be investigating whether an explosive could have been concealed in the fruit.
"This is yet another theory," an unnamed police officer told the Daily Mail.
"But we are not discounting anything."
- Sydney Morning Herald