Officials today confirmed what we have feared for some time - that a relatively tiny search zone in the southern Indian Ocean is not the final resting place of Malaysia Airlines flight MH370.
From an underwater mission covering 850 sq km (320 sq mile) where acoustic "pings" were heard, the area being searched has now been extended to around a 60,000 sq km (23,100 sq mile) zone based on satellite data which remains disputed in some quarters.
The Australia-led search control team estimate it could be August - next year - before this region has been covered, and hopes of finding the Boeing 777's flight recorders are becoming increasingly dim.
With so much uncertainty surrounding the circumstances of MH370's bizarre disappearance, it has become rich territory for aviation experts, bloggers and conspiracy theorists alike.
Here we round up 13 of the most prominent theories as to where the plane ended up, and what went wrong in the first place.
Shot down in a military training exercise
While the Australian officials leading the search for MH370 say they remain “absolutely convinced” it ended up in the southern Indian Ocean, some passengers’ families – and theorists – distrust the unprecedented satellite data analysis involved.
Among those who support this view are the British journalist and author Nigel Cawthorne, who has controversially already published the first book on the plane’s disappearance.
He supports one theory, based on the eye-witness testimony of New Zealand oil rig worker Mike McKay, that the plane was shot down shortly after it stopped communicating with air traffic controllers.
At the time there was a series of war games taking place in the South China Sea involving Thailand, the US and personnel from China, Japan, Indonesia and others, and Cawthorne has linked this to Mr McKay’s claims to have seen a burning plane going down in the Gulf of Thailand.
Flown north and shot down deliberately, prompting cover-up
At a stage in the investigation when it was believed the plane could have flown for some time from where it disappeared along either a northern or southern corridor, many posted on forums suggesting that if it had been the former we would never hear about what happened.
Some still support this view, and former RAF navigator Sean Maffett told the BBC that after 9/11, any unidentified airliner entering the airspace of another country would lead to fighter jets being scrambled.
“If the plane is in the northern arc it could easily have been shot down,” he said. This theory also involves a national – or possibly international – cover-up, based on the premise that no country would want to admit to shooting down an airliner full of passengers from all over the world.
Flown north in the ‘shadow’ of another plane
Another theory suggests that instead of flying south, the plane flew north in the “shadow” of another airliner around half an hour to an hour after dropping off civilian radar.
The aviation blogger Keith Ledgerwood argued that MH370 and Singapore Airlines flight 68 were in the same vicinity at the time, and said: “It became apparent as I inspected SIA68's flight path history that MH370 had manoeuvred itself directly behind SIA68 at approximately 18:00UTC and over the next 15 minutes had been following SIA68.”
By flying a short distance behind and most likely a little above the altitude of SIA68, also a Boeing 777, Ledgerwood said that it would be able to appear as a single blip on radar screens.
SIA68 flew on to Spain – and this theory suggests MH370 could have branched off and landed in one of a number of locations across Xinjiang (north-east China), Kyrgyzstan or Turkmenistan.
Experts have said that the idea sounds “feasible”, and that even if higher-resolution military radar was monitoring SIA68 operators might have dismissed the fact that there were two objects as an technical glitch or echo.
Tried to land on a desert island beach
After reports that the plane had turned left shortly following its disappearance from civilian radar screens, speculation grew that it could have landed on a remote beach somewhere like the Andaman Islands, which lie between Indonesia and the coast of Thailand.
Though CNN reported that locals dismissed the idea a Boeing 777 could land on an airstrip there undetected, the archipelago consists of hundreds of remote islands with some long stretches of sand.
Former BA pilot Steve Buzdygan said it would be difficult – but not impossible – to bring a 777 down on a long deserted beach.
Landed at a US military base
One of the more outlandish conspiracy theories that has gained some traction online is the idea that MH370 could have been “captured” and flown to a military base on the UK-owned tropical atoll of Diego Garcia, in the middle of the Indian Ocean.
The base is run by the US, and some have reportedly said in forum postings that the Kremlin has put some credence into this possibility.
Such is the strength of belief in this theory that the US government has been forced to issue a denial. A spokesperson for the US embassy in Malaysia told the local Star newspaper that there was “no indication that MH370 flew anywhere near the Maldives or Diego Garcia”. “MH370 did not land in Diego Garcia,” he added.
Headed for a remote airport in Langkawi, Malaysia
One theory, put forward by another aviation blogger named Chris Goodfellow, has it that the sudden left turn came after major catastrophe knocked out a range of the plane’s electronics, from transponders to communications equipment.
In this scenario and in the middle of the night, Goodfellow argued, the pilot would redirect towards the nearest safe airport.
“This pilot did all the right things,” he said. “Actually he was taking a direct route to Palau Langkawi, a 13,000ft (4,000m) strip with an approach over water at night with no obstacles. He did not turn back to Kuala Lumpur because he knew he had 8,000ft ridges to cross. He knew the terrain was friendlier towards Langkawi and also a shorter distance.”
This theory assumes that the plane was in fact controlled manually once it disappeared – and that it did not make it to Langkawi.
A fire throughout the plane
Many theories accept that the Inmarsat satellite analysis is accurate – that the plane headed south into the Indian Ocean and flew on for hours before a final, partial “handshake” in a remote location thousands of miles off the west coast of Australia.
The issue here becomes explaining what happened in the cabin between the last contact with flight controllers and the plane’s seemingly inevitable crash far out to see.
One suggestion is that a fire broke out, not just in the cockpit but throughout the interior of the plane. The implication is that this resulted in the attempt to turn back, after which the fire killed those on board.
This theory would then have it that the fire went out before damaging the exterior of the plane, which flew on autopilot until its fuel ran out.
Yet such a fire would be expected to spread with at least some warning – and that surely would have given the pilots time to issue a mayday distress signal.
An explosion in the cockpit
The theory of a sudden explosion within the cockpit before the turn left could explain why there was no attempt to signal for help.
Since 9/11 cockpits doors have been fortified to become extremely difficult to bypass, and such a sudden incident could perhaps have incapacitated both pilots while keeping out the rest of the crew.
This explanation does not seem to tally with the claims of some Malaysian officials, however, that the change in direction was the result of “seven or eight keystrokes into a computer on a knee-high pedestal between the captain and the first officer”.
A struggle at altitude
Though Malaysian officials believe that the plane was deliberately diverted, and that its communications systems were turned off one after the other, a detailed background check into all 227 passengers has cleared all of suspicion.
If, however, we do accept that the plane was the subject of a passenger hijacking, it remains to be explained why the hijackers did not try to do more than fly the plane into the middle of the southern Indian Ocean.
One theory suggests that there was some kind of struggle for control of the plane that ultimately ended with mutual destruction.
Further analysis of data by Malaysian officials suggests that the plane was flown erratically once it left civilian radar, climbing to 45,000ft before dropping very low. Buzdygan told the BBC he would resort to this sort of flying if faced with would-be hijackers. “I’d try to disorientate and confuse the hijackers by throwing them around,” he said.
A botched hijack attempt
The climb to 45,000ft could also have been carried out by the hijackers once they had taken control – in a bid to kill the passengers on board.
At such an altitude it could be possible to depressurise the cabin, causing oxygen supplies to be deployed. These run out after 12-15 minutes and, if those flying the plane had access to another oxygen supply, could have been an attempt to prevent anyone intervening.
Under this theory the suggestion is clearly that the attempt failed, killing the hijackers as well.
As part of the ongoing criminal investigation in Malaysia, police are looking into the state of mind and possible motives of the captain Zaharie Ahmad Shah and co-pilot Fariq Abdul Hamid.
The Malaysian police chief Khalid Abu Bakar has said that “all possibilities” will be looked into, and there have been reports that Shah was going through a difficult marriage break-up.
Yet such comments have been rubbished by the man’s relatives, who have described him since as a dedicated family man and model professional.
Hugh Dunleavy, the commercial director of Malaysia Airlines, described Shah as a seasoned pilot with an excellent record.
“There have been absolutely no implications that we are aware of that there was anything untoward in either his behaviour or attitude,” he told Reuters. “We have no reason to believe that there was anything, any actions, internally by the crew that caused the disappearance of this aircraft.”
Sabotage – for a life insurance scam or corporate attack
One of the other strands of the criminal investigation regards whether the plane was subject to some form of sabotage – either as part of a life insurance scam or over industrial espionage.
Bakar said that when passengers and crew were being investigated, police were looking for “Maybe somebody on the flight has bought a huge sum of insurance, who wants family to gain from it or somebody who has owed somebody so much money, you know, we are looking at all possibilities.”
There were also 20 employees of the US silicon chip company Freescale Semiconductor on board the plane at the time, and a retired Delta Airlines pilot has suggested the plane’s disappearance was an attempt to steal technology the engineers had applied – but not yet received – a patent for.
A CIA cover-up
Finally, the former prime minister of Malaysia Mahathir Mohamad has waded in with his own theory – suggesting that, one way or another, the CIA is definitely hiding something.
In a blog entry posted on 18 May entitled ‘Boeing Technology – What goes up must come down’, Dr Mahathir Mohamad makes ten claims including that the plane was taken over remotely by officials working for Boeing and the CIA.
“The plane is somewhere, maybe without MAS markings,” reads Dr Mohamad’s post on chedet.
“Someone is hiding something. It is not fair that MAS and Malaysia should take the blame,” 88-year-old Dr Mahathir, who was Malaysia's prime minister between 1981 and 2003, alleges.
“Airplanes don’t just disappear,” he said, concluding: “For some reason the media will not print anything that involves Boeing or the CIA. I hope my readers will read this.”
Boeing have denied Dr Mohamed’s theory.
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