Logical, sinister or downright bizarre: here are some of the theories that have circulated over the years.

The pilot did it

Zaharie Ahmad Shah, 53, had been a pilot with MAS since 1981. He captained the Boeing 777 for more 15 years and had 18,365 hours of flight experience under his belt.

On March 8, 2014, he and co-pilot Fariq Ab Hamid were supposed to fly MH370 from Kuala Lumpur International Airport (KLIA) to Beijing’s Capital International Airport. When the plane was reported missing, speculation ran rife that Zaharie was somehow involved.

One theory suggested that the veteran pilot had hijacked the flight, intentionally taking MH370 off the official route. This corresponded with the discovery that he had tracked a course into the Indian Ocean on his home flight simulator that closely matched the suspected route of the missing plane. The simulated flight had been made less than a month before the plane disappeared.

Why Zaharie might have done so triggered further theories. According to one, it was a case of suicide. Following the flight’s disappearance, unsubstantiated reports said that Zaharie, an opposition supporter, was upset over a jail sentence handed to then-opposition leader Anwar Ibrahim. The sentence had been announced hours before the plane took off.

Other reports said Zaharie had been struggling with personal problems. These reports claimed Zaharie was heartbroken over a split with his wife three weeks earlier. It is believed that he and his wife were separated, although they still lived in the same house. Zaharie’s family has always rejected these claims.

Some accounts painted Zaharie as a hero, saying the pilot had crashed the plane into the sea after a fire broke out on board in order to avoid causing mass casualties on land.

Flash fire

Soon after MH370 disappeared, there were reports claiming that a fire had brought down the plane. These were largely based on a US Federal Aviation Administration airworthiness directive regarding the oxygen systems on Boeing 777s. On some 777s, low-pressure oxygen flex hoses could be conductive, leading to oxygen fires on board, it said. However, this theory was debunked by aviation experts who pointed out that MH370 had not used the oxygen system in question.

Others said an electrical fire could have caused enough smoke for the pilot and crew to have passed out from smoke inhalation. The plane would then have continued flying for hours on auto-pilot until it ran out of fuel and crashed. This theory, too, has been largely dismissed due to a lack of solid evidence.

Rapid decompression
In 2016, aviation journalist Christine Negroni released a book titled “The Crash Detectives” in which she theorised that an explosive decompression had sucked the air out of the cabin, depriving those on board of oxygen. She said Zaharie could have stepped out of the cockpit for a bathroom break, leaving his less experienced co-pilot to fly the plane.

If MH370 indeed crashed due to rapid decompression, it is likely that Zaharie and Fariq were not responsible. In an interview with a commercial long-haul pilot for a major airline soon after MH370 vanished, Britain’s The Guardian was told that it was impossible for a pilot to force decompression and to prepare for it.

Terrorist hijacking

News that two of the passengers had boarded MH370 using stolen passports quickly sparked rumours of a terrorist takeover. Iranians Pouria Nour Mohammad Mehrdad, 18, and Delavar Seyed Mohammadreza, 29, were travelling under the names of Luigi Maraldi and Christian Kozel.

The two were said to be seeking asylum in Europe and, before boarding the flight, had stayed with a fellow Iranian in Kuala Lumpur known only as Mohammad. It is believed that the passports were stolen from Maraldi and Kozel in Thailand the year before.

Mohammad later told the British Broadcasting Corporation (BBC) that the young men had appeared nervous. However, he maintained that the two were not terrorists, adding that they were “nice, never naughty”.

Authorities rejected suspicions of a terror link once the identities of the two were established. However, theories remained that MH370 had been hijacked and was being prepared for use in a terrorist attack timed for the anniversary of the 9-11 attacks on New York’s World Trade Centre and the Pentagon.

MH17 switch

According to one of the more bizarre theories circulating on the internet, it was MH370, not MH17, that was shot down over Ukraine later that year. Little has been offered to explain how the planes were switched and what happened to the real MH17, but those who back the theory claim that MH370/MH17 was shot down as part of a Mossad plot to hide “the dirty business” going on in Gaza.

Alien abduction

Still others say “the truth is out there.” According to a CNN survey taken in 2014, 9% of Americans polled believed that aliens or beings from another dimension were involved in the plane’s disappearance.

Soon after MH370 went missing, one of CNN’s own, Don Lemon, came under fire for entertaining the idea that extra-terrestrials were connected to the disappearance. The news anchor had suggested that MH370 was sucked up by a black hole.

In March 2016, an alien hunter claimed to have found MH370 off the coast of Cape Town, South Africa. However, nothing came of his report, which was published on a UFO sightings website.

Related theories include the idea that an “Asian Bermuda Triangle” could have swallowed up MH370, recalling theories about the disappearance of boats and planes in the Bermuda Triangle in the North Atlantic Ocean.

Military op gone wrong

A 2014 book titled “Flight MH370: The Mystery”, written by Briton Nigel Cawthorne, suggested that the airliner had been shot down by US-Thai strike fighters in a training drill gone wrong.

According to the book, the drill involved mock warfare on land, on water and in the air. It also claimed that the drill included live-fire exercises.

Cawthorne said he did not believe the theory himself, but he was nonetheless slammed by relatives of passengers who said the book was insensitive.

Other theories connecting MH370 with army operations said the plane had landed on the island of Diego Garcia, which is home to a major US military base. The idea became so popular that US embassy officials were forced to issue a statement denying it.

However, those who subscribe to the Diego Garcia theory claim it explains why the passengers’ mobile phones were still ringing in the hours following MH370’s disappearance.

We may never know

Over the years, scores of theories have emerged on the mystery of MH370. The world has never been more connected, information has never been more readily available, and yet we are no closer to solving the mystery than we were three years ago.

Dramatic theories may abound, but aviation experts say the truth is mundane: despite our best guesses, we may never really know.