IT is the greatest aviation mystery the world has ever known, but the search for missing Malaysia Airlines flight MH370 ended this week after more than four years ... and there are still no answers.

The disappearance of the Boeing 777 on March 8, 2014 gripped people across the globe. The flight took off from the Malaysian capital Kuala Lumpur bound for Beijing, China, with 239 passengers and crew on board. Major television networks immediately started following the breaking story, and all manner of theories were explored as the search went from hours to days, and then to weeks and months.

Investigators and experts from seven nations eventually joined the search for the plane, pooling together technology and resources to find every last scrap of data about the aircraft’s last known whereabouts. Investigators discovered that the plane disappeared around an hour into the flight, shortly after a key piece of equipment, a transponder, was mysteriously switched off.

Even more mysteriously, it then emerged that the plane’s flight path was changed, taking it on a sharp westward turn. The combination of strange developments heightened fears that MH370’s disappearance was a malicious act, but even as more information about its final hours emerged, answers were still far out of reach.

For those around the world who have followed the search for MH370 with fascination, the news that the search for the plane has now ended without solving the mystery is disappointing. The “official” search for the plane ended last year, but private firm Ocean Infinity, based in the US, continued looking with a deep-sea vessel in the Indian Ocean. While it funded the search on its own, Malaysia agreed an award of $70m if it found the wreckage.

And despite the location of the missing plane remaining a mystery – the search area in the Indian Ocean spans an area the size of the United States – there was hope of finding it after debris belonging to MH370 washed ashore on the island of Reunion, east of Madagascar, in 2015. However, there has been a lack of further major developments, and the search has finally been called off.

The search for MH370 prompted many to veer off on their own investigations. Some kept blogs outlining evidence, others released books. Multiple theories have been put forth since 2014, and the story still sparks huge interest among the public. Here, we take a look at some of the most likely, and some of the plain weird, explanations for the disappeared MH370.

TERRORISM

What’s the theory?

One of the immediate concerns when the plane went missing was that it could have fallen victim to a terrorist attack. In the days after its disappearance, it emerged that two passengers were travelling with false documents, heightening concerns of foul play. There are also active Islamist terror groups in the Malaysia region.

What’s the problem?

Authorities soon revealed that one of the Iranian passengers was an asylum seeker and that it was common for Iranians seeking asylum to travel through Malaysia in a bid to get to Europe. All investigations so far have concluded it is unlikely that terrorism was a factor in MH370’s fate.

FIRE

What’s the theory?

One of the key questions in the MH370 story is why the plane’s transponder – which sends and receives radar signals – was switched off. If it was deliberately switched off it raises concerns about human motive. However, a fire onboard the plane could have caused the system to disconnect, and could have contributed to a catastrophic mid-air event that eventually brought the plane down.

What’s the problem?

Neither the pilot nor co-pilot made any distress call before the plane disappeared from radar systems. MH370 also continued sending "pings" – electronic location messages – to satellite systems for hours after it went missing. These pings did not offer detailed information about the status of the plane or whether it was in trouble, but the information was enough to confirm that the plane carried on flying for a substantial period – six hours – without any contact from pilots. If a fire was strong enough to knock out systems and potentially incapacitate the crew, it’s unclear how the plane was in a fit enough condition to continue flying for so long.

PILOT SUICIDE

What’s the theory?

Pilot murder-suicide shocked the world when Germanwings Flight 4U 9525 was deliberately crashed into the Alps in 2015, killing all on board. The flight was travelling from Spain to Germany when 28-year-old co-pilot Andreas Lubitz locked the pilot out of the cockpit and crashed the plane. The aircraft's black box recorded the crew's desperate attempts to break the secure cockpit door down in the minutes leading up to the disaster. While it is unusual for a pilot to commit suicide in this way, it is not unheard of. In a new book, Canadian accident investigator Larry Vance argues that MH370 was deliberately flown into the Indian Ocean. After the plane went missing it emerged that pilot Zaharie Ahmad Shah was having marital problems, raising concerns about his state of mind.

What’s the problem?

While pilot murder-suicide is one of the leading theories, some investigators remain convinced that a catastrophic accident or mechanical malfunction is more likely to have caused the plane to crash. Murder-suicide is a very rare event.

HYPOXIA

What’s the theory?

Hypoxia occurs when the aeroplane cabin becomes depressurised. This can be caused by something explosive, like a bomb, or it can happen more gradually due to a technical fault. Hypoxia reduces the oxygen available to passengers and crew, and will cause oxygen masks to drop. However, they only last for a relatively short period of time. When suffering from hypoxia, passengers and crew would likely become confused and disorientated before falling asleep, leading quickly to death. In 2005, Helios Airways Flight 522 crashed after passengers and crew passed out due to hypoxia. The plane’s autopilot meant that the plane continued flying, and the "ghost flight" was discovered when two F-16 fighter jets were scrambled alongside it to explore why communications had stopped.

What’s the problem?

Two key points are stumbling blocks for the hypoxia theory: the plane’s transponder was switched off during the flight, and it changed course. Investigators believe both of these actions were likely the result of deliberate human intervention. If a mechanical error caused a hypoxia event that the crew were aware of, it’s possible the pilot may have tried to change course in order to land the plane. However, the lack of any indication from crew that there was a problem leaves investigators perplexed.

THE WEIRD THEORIES

ALIENS

What’s the theory?

When in doubt, blame aliens. Some internet websites have proffered the idea that MH370 may have been abducted by aliens for some unknown experiment.

What’s the problem?

No aliens have been in touch with any ransom demands or other supporting evidence.

BLACK HOLE

What’s the theory?

After prolonged broadcast TV coverage, CNN appeared to briefly lose its senses when an anchor once questioned experts on the possibility of a black hole consuming MH370.

What’s the problem?

One of the experts on the programme, Mary Schiavo, formerly of the US Department of Transportation, said: “A small black hole would suck in our entire universe, so we know it’s not that.”

MH17

What’s the theory?

Some conspiracy theorists believe that two Malaysia Airlines disasters in close proximity to one another is too coincidental. Months after MH370 went missing, another flight, MH17, was shot down over Ukraine amid tensions in the region with Russia. The theory suggests that it was actually MH370 that was shot down in Ukraine in a bid to cover something up.

What’s the problem?

As with all conspiracy theories, it can be hard to disprove the motive of "covering something up" when, naturally, nobody knows what the thing being covered up actually is. However, we do know that these flights involved hundreds of passengers, they had crews and airport staff working across a range of services, including security. That’s before the eventual involvement of air crash investigators, aviation experts and multiple international authorities taking an interest. It would be very difficult to conceal such an elaborate conspiracy with so many players involved.

AN UNLIKELY EXPERT

Oscar-winning Titanic director James Cameron got involved in the mystery last week when he offered his view that it’ll be a long time before the remains of MH370 are found. Cameron, who actually does have a bona fide interest in deep-sea exploration and has researched sunken shipwrecks, told an Australian news website that investigators don’t yet possess the kind of technical equipment that would likely be needed to locate the aircraft. “We know more about the surface of Mars or the Moon than we do our deepest oceans,” he said, prompting a number of amused social media users to request that the search for MH370 be formally handed over to Cameron.