The 239 people on board were never found, and are presumed dead.
Investigations dedicated to finding the plane, the victims, or any substantial evidence of what really happened, have come up with next to nothing, inviting speculation and conspiracy to fill the vacuum.
This Monday, the Malaysian government, which has taken chief responsibility for investigating the disappearance, will issue what it claims is the final report on the fate of the plane.
Transport minister Anthony Loke has promised total transparency in the document, with no redactions or obfuscations — but it's unclear what the authorities could say to truly draw a line under the disaster.
Here's what we know, what theories have been put forward and the unanswered questions the report might be able to address.
Malaysia Airlines flight 370, a Boeing 777, left Kuala Lumpur at 12:41 a.m. local time on March 8, 2014. 239 people were on board.
The majority — more than 150 — were Chinese. The rest included 38 Malaysians, seven Indonesians, six Australians, and individuals passengers from a host of other countries including France and the US. Twelve crew members were on board as well.
The final voice communication from the cockpit came less than an hour after takeoff: "Good night Malaysian three seven zero."
The plane then stopped communicating with ground control.
It deviated from its planned route two hours after takeoff, according to data from military radar.
For reasons that are still unknown, the plane abruptly left its planned route and turned back in the direction of Malaysia, then on towards the Indian Ocean. This map shows where it went before dropping off the grid:
There were no reports of bad weather or distress calls.
The plane traveled south and may have changed altitude at some point. The plane sent out a final, automated satellite communication at 8 a.m., seven hours and 30 minutes after it took off. The signal was received but did not contain any information about where the plane was.
Other aircraft communications had been purposefully disabled earlier in the flight, authorities said. The plane had disappeared.
Search attempts were quickly launched, but with little idea of where to start, and a vast swathe of territory where the plane could feasibly be.
Teams from Singapore, Vietnam, and Malaysia were searching the waters off Vietnam — which is partway between Kuala Lumpur and Beijing — around 12 hours after contact was lost.
FBI team and Interpol personnel flew to assist the operation, with a total of nine contributing to the initial search.
Forty ships worked around the clock and 34 aircrafts flew during daylight hours. A total of 26 countries pitched in, as the search area was repeatedly widened. They found nothing.
The next step began on 5 May 2014, 79 days after the plane went missing.
Malaysia, Australia, and China announced their agreement to hold an underwater search. At the request of the Malaysian government, the investigation was led by the Australian Transport Safety Bureau. But it discovered little.
Malaysia officially declared the disappearance to be an accident in January 2015 – 327 days after it vanished.
A report one year after the disappearance gave a detailed picture of delays and protocol violations before the search was launched.
It revealed that five hours and 13 minutes passed between the last communication from the plane and Kuala Lumpur's first distress signal about the disappearance, and that another five hours passed before the first search flights took off to try and find it.
The first actual piece of MH370 took more than a year to find. It was a fragment of the plane's wing, which washed up in July 2015 on Réunion island, thousands of miles from Kuala Lumpur.
After a four-month hiatus to allow for detailed mapping of the sea bed, the search resumed in October 2015, but bad weather continued to slow the investigation.
The two-year anniversary report in March 2016 shed no more light on what had happened.
By May 2016 more than 105,000 sq km (more than 65,240 miles) of seafloor in the southern Indian Ocean had been covered, leaving just 15,000 sq km (more than 9,320 miles) left to look at.
In July 2016, the Dutch company hired by Australia to lead the search said it may have been looking in the wrong part of the sea for the past two years.
Families became so frustrated that they launched their own organized search for debris in Madagascar.
The fruitless search was suspended in January 2017, after $145 million was spent. Officials then concluded that the probable crash site was farther north.
In the year that followed, organizations like Australia's chief science agency released reports while analysis of satellite imagery and debris continued as talks to formally begin a new search continued.
The Malaysian government started the latest search in January 2018 in partnership with US-based private firm Ocean Infinity. The search covered 112,000 sq km (43,243 sq miles) in the southern Indian Ocean, including a 25,00 sq km (15,534 sq mile) target area that had not been searched before. It was called off on May 29 with no new significant new findings, even though investigators still wanted to search further north.
In four years of searching, the only confirmed traces of the Boeing 777 aircraft have been three wing fragments washed up on Indian Ocean coasts: in Madagascar, Réunion Island, and Tanzania.
Prime Minister Mahathir Mohamad has said Malaysia would consider resuming the search if new clues came to light.
The lack of concrete information, combined with the fruitless search efforts, means all that is left are theories of what might have happened to the flight — some more plausible than others.
The theory adopted by the Malaysian government and the Australian Transport Safety Bureau, is that the passengers and crew of MH370 were incapacitated by an oxygen deficiency.
This theory speculates that the whole plane, including captain Zaharie Ahmad Shah, somehow fell unconscious. The plane would have run on autopilot after it ran out of fuel, before crashing into the sea. Versions of this theory include a deliberate hijacking in which someone damaged the oxygen supply or some kind of accident that harmed the plane’s mechanics and affect the plane's oxygen levels.
In another version of this theory, Shah was the only one awake. In a suicidal state, he disabled communication before steering the plane into the ocean. A team of analysts on Australian current affairs programme “60 Minutes” came to this conclusion.
Others also support the theory that the act was a deliberate one by the pilot. A Canadian aviation expert and former airplane crash investigator told Canada's CBC news in May 2018 that he could state with "100 per cent certainty" that the incident was caused by a pilot in a murder-suicide.
But the idea that the pilot was responsible was refuted by the official in charge of the search.
A former pilot Christopher Goodfellow speculated that the plane had experienced an electrical fire and turned back towards Malaysia to try to find an emergency landing strip.
According to Goodfellow, the plane would have flown on autopilot as the fire incapacitated the crew and passengers.
But another pilot, Patrick Smith, has cast doubt on this theory, as it would have been unlikely that the plane would travel for hours on autopilot after a fire. The plane also turned back for a second time after it headed towards Malaysia.
Smith told Business Insider in 2015 that the nature of the debris found might suggest that the airline was under control when it crashed.
The plane may have landed gently into the water, and sunk in mostly one piece. Experts have previously said that this theory is consistent with the debris found. This would mean that the plane had a low-energy crash or else a gentle, deliberate landing in the ocean.
Another theory is that the search has taken place in the wrong area – investigators should be looking north of Malaysia, rather than Australia. Perhaps, this theory suggests, the plane crashed or even landed and was hidden somewhere. But it was rejected by Inmarsat, the British company that owned and operated the satellite that tracked MH370 until it dropped off the network.
Other theories claimed that the plane was taken over “remotely” in a bid to foil a hijack. Malaysia’s prime minister Mahathir Mohamad suggested this in May 2018, but did not state who he believed had done it.
The details that first emerged as the plane’s disappearance was reported – from the fact that two people boarded with stolen passports and no one seemed clear if everyone had boarded the plane — meant that conspiracy swirled. Conflicting accounts from officials and families accusing the Malaysian government of withholding information added fuel to this fire.
More outlandish theories include a remote cyber hijacking, the plane being shot down, the idea that Russian president Vladimir Putin knows the plane’s location, a mystery extra passenger taking control of the plane, a Bermuda Triangle-style area causing the plane’s disappearance, North Korea taking the plane, and the US military shooting it down.
Other theories blame the CIA, Israel, and aliens, all traditional boogey men for conspiracists.
So much information remains unknown, despite the efforts of the largest and most expensive underwater search in history. But Monday's report may offer something new.
If the plane's route can be established, or the plane itself is found, it could provide clues as to whether the plane was deliberately heading somewhere or an if an accident took place.
More basic information is still not known. We don't know if there was a case of oxygen deprivation or if there was an accident on board or it was a deliberate action. We don't know which one of the pilots was flying the plane when it disappeared, or whether it was someone else, which would help answer the question of whether the incident was a hijacking.
It may be that even if the plane is found, these questions would remain unanswered.
Relatives of passengers hope that this report will include all details, as promised. Voice 370, a group representing the relatives, has pressed the new Malaysian government to review everything related to MH370, including "any possible falsification or elimination of records related to MH370 and its maintenance," the news agency Reuters reported.
Calvin Shim, whose wife was a crew member on the plane, said he was concerned that accident report would not include key information, such as a full description of the plane’s full cargo and the results of a separate investigation by Malaysian police.
Monday's report may offer new answers, or it might increase the likelihood that the fate of MH370 might never be known.