More than 1000 days were spent looking for missing flight MH370 with no success, but authorities say they now have a better understanding of where it might be.
The Australian Transport Safety Bureau on Tuesday published its final 440-page report into the search, from the time the Malaysia Airlines plane disappeared on March 8, 2014, until it was suspended in January.
"We ... deeply regret that we have not been able to locate the aircraft, nor those 239 souls on board that remain missing," the report said.
It was the largest search of its type in aviation history, covering several million square kilometres of the ocean's surface and below.
"Despite the extraordinary efforts of hundreds of people involved in the search from around the world, the aircraft has not been located," the report said.
It acknowledged that it was "almost inconceivable and certainly societally unacceptable", in an era where 10 million passengers fly daily, for a large commercial aircraft to be missing.
"And for the world to know with certainty what became of the aircraft and those on board," it said.
Nevertheless, the ATSB said that having eliminated most of the high probability areas of its location, new studies in the past 12 months have identified the most likely area with increasing precision.
A re-analysis of satellite images from 2014 has pinpointed an area of less than 25,000sq/km that has the highest likelihood of housing MH370.
"The understanding of where MH370 may be located is better now than it has ever been," it said.
The Malaysian government is continuing work on its investigation of the facts and circumstances surrounding the loss of MH370, according to the report.
But, it conceded, the reasons for its disappearance cannot be established with certainty until the aircraft is found.
The ATSB said the incident has led to some important learnings about locating missing aircraft on flights over deep ocean areas, with improvements to the requirements and systems for tracking aircraft.
"Steps are being taken to advance other aircraft systems including emergency locator transponders and flight recorder locator beacons."
ATSB chief commission Greg Hood praised the commitment and professionalism of everyone involved in the search.
"This was an unprecedented endeavour and there has been an extraordinary response from the global community," he said in a statement.
THE SEARCH BY NUMBERS
* 1046 days spent searching
* 710,000sq/km of Indian Ocean seafloor mapped, the largest ever single hydrographic survey
* 120,000sq/km of high-resolution sonar, also the largest ever search or survey of its kind
* 661 areas of interest identified in sonar imagery of the seafloor, 82 of which were investigated and eliminated as being related to MH370
* Four shipwrecks identified in the area searched