Malaysia Airlines flight MH370 took off from Kuala Lumpur heading for Beijing on March 8, 2014 — and vanished.
Now, nearly 12 years later, a new search is set to begin.
Is the new search likely to find new evidence more than a decade after the flight went missing? And with satellite technology how does a commercial aircraft disappear? This is what we know.
'Goodnight, Malaysian three seven zero'
The disappearance of flight MH370 with 239 people on board remains one of the world's most baffling aviation mysteries.
The Boeing 777 went missing on its way from Kuala Lumpur to Beijing on March 8, 2014.
There were seven Australians, more than 150 Chinese and 50 Malaysians on the flight, as well as citizens of France, Indonesia, India, the United States, Ukraine and Canada.
Satellite data analysis showed the plane most likely crashed somewhere in the southern Indian Ocean, off the coast of Western Australia.
However, two major searches failed to come up with any significant findings.
The last radio transmission from the plane was about 40 minutes after it took off.
Captain Zaharie Ahmad Shah signed off with "Goodnight, Malaysian three seven zero" as the plane entered Vietnamese airspace.
Soon after, its transponder was turned off, which meant it could not be easily tracked.
Military radar showed the plane left its planned flight path to fly back over northern Malaysia and Penang Island and then out into the Andaman Sea towards the tip of the Indonesian island of Sumatra.
It then turned south and all contact was lost.
Could new tech find lost jet?
A deep-sea hunt for the plane was planned to start on Monday.
The search will be carried out by US-based marine robotics company Ocean Infinity, which signed a new "no find, no fee" contract with the Malaysian government, which agreed to pay $100 million — but only if the wreckage is discovered.
Charitha Pattiaratchi, a professor of coastal oceanography at the University of Western Australia, has been involved with the search for MH370 since 2014.
Ocean Infinity is expected to search for 55 days in a remote part of the southern Indian Ocean and will use technology that didn't exist during earlier attempts.
Professor Pattiaratchi told Radio National that Ocean Infinity had conducted a search several years ago from ship Seabed Constructor, using autonomous underwater vehicles to search parts of the seabed in the southern Indian Ocean.
"Now they have gone up a notch ... and they have built new ships, which are much more efficient," he said.
"Also, the technology in the sonar and analysis has improved."
Ocean Infinity will be searching for large pieces of debris, Professor Pattiaratchi said, like engines and other big, heavy parts of the aircraft that are more likely to still be intact.
"It took almost 100 years to find the Titanic and they did know where it went down, so it is not an easy challenge," he said.
Loading...How did earlier MH370 searches go?
Malaysia, Australia and China launched an underwater search of 120,000 square kilometres of the southern Indian Ocean, based on data from automatic connections between a satellite and the plane.
The search, which cost about $200 million, was called off after two years in January 2017, with no trace of the plane found.
In 2018, Malaysia accepted a "no cure, no fee" offer from Ocean Infinity for a three-month search.
That search covered 112,000 square kilometres north of the original target area and also proved fruitless, ending in May 2018.
What has been found?
More than 30 pieces of suspected aircraft debris have been collected along the coast of Africa and on islands in the Indian Ocean, but only three wing fragments were confirmed to be from MH370.
Most of the debris was used in drift pattern analysis in the hopes of narrowing down the aircraft's possible location.
A 495-page report into MH370's disappearance, published in July 2018, said the Boeing 777's controls were likely deliberately manipulated to take it off course, but investigators could not determine who was responsible.
The report also highlighted mistakes made by the Kuala Lumpur and Ho Chi Minh City air traffic control centres and issued recommendations to avoid a repeat incident.
Investigators stopped short of offering any conclusions about what happened to MH370, saying that it depended on finding the plane's wreckage.
MH370 conspiracy theories
The inability to locate MH370's crash site has fuelled conspiracy theories ranging from mechanical error or a remote-controlled crash, to more bizarre explanations like an alien abduction, an organ-harvesting ring, a crash in the Cambodian jungle and a Russian plot.
In recent years, some aviation experts have said the most likely explanation was that the plane was deliberately taken off course by an experienced pilot.
But investigators have said there was nothing suspicious in the background, financial affairs, training and mental health of either captain Zaharie Shah or second pilot Fariq Abdul Hamid.
Neither had any apparent motive for intentionally taking the plane off course.
Another theory was that Boeing had filed a US patent in February 2003 for a system that could remove all controls from pilots and allow remote operators to fly and land an aircraft at a predetermined location to foil hijacking attempts.
But Boeing confirmed in 2018 it had never installed such a system on any aircraft.
In early 2018, when Ocean Infinity was searching for MH370 in the Indian Ocean, it also became the subject of a conspiracy after the Seabed Constructor turned off its Automatic Identification System for several days, preventing online observers from tracking its movements.
No evidence of terrorism or fire has been found on any of the recovered debris and no terrorist group has claimed responsibility.
Theories about what happened range from hijacking to cabin depressurisation and power failure.
There was no distress call, no ransom demand, no evidence of technical failure or severe weather.
The families of the 239 people onboard Malaysia Airlines flight MH370 have been searching for answers since the plane went missing.
ABC, Reuters