The hunt for Flight MH370 has failed to turn up any debris, but its unprecedented scale in one of the world's remotest locations has provided valuable lessons for future search and rescue missions.
The Malaysia Airlines plane disappeared en route from Kuala Lumpur to Beijing on March 8, 2014, with mourning families and friends of the 239 people on board still waiting to hear what happened 12 months later.
There has been no trace of the Boeing airliner despite an extensive air and sea search.
Four ships, coordinated by Australia, continue to scour a huge underwater area at least 1,600 kilometres (1,000 miles) from the nearest piece of land in a stretch of the Indian Ocean previously only mapped by satellite.
"The size of the area we're covering is unprecedented," search chief Martin Dolan, head of the Australian Transport Safety Bureau, told AFP.
"At most, when the French were looking for Air France 447, it covered a quarter of the sort of area we have in mind."
Flight AF447 was hauled from the Atlantic nearly two years after it crashed in 2009 en route from Rio de Janeiro to Paris.
The MH370 search -- jointly funded by Australia and Malaysia with a budget of Aus$120 million (US$93 million) -- is focused on a 60,000 square kilometre (23,000 square mile) priority area and is scheduled to end in May.
The thin, long stretch of water is within the so-called seventh arc, where the plane was calculated to have emitted a final satellite "handshake".
After months of painstaking efforts, which have seen the ocean floor in the area mapped for the first time, all authorities have found is a handful of shipping containers.
Remote, treacherous conditions
The search area is so remote that the four vessels involved -- Fugro Supporter, Fugro Equator, Fugro Discovery and GO Phoenix -- need up to six days to reach it from the Australian port of Fremantle, where they routinely refuel and restock.
While at sea, they frequently encounter conditions similar to the "Roaring Forties" north of Antarctica, winds that whip up mountainous seas.
The turbulence penetrates below the ocean surface, buffering the 10-kilometre (six-mile) long tow cables extending into the water with sophisticated sonar systems attached.
A relative of one of the Chinese passengers from the missing Malaysia Airlines flight MH370 holds a candle before offering prayers at Thean Hou Temple in Kuala Lumpur on March 1, 2015.
The systems scour the never-before studied ocean floor in pitch-black conditions, with the plane believed to have sunk to depths of 4,000 metres (13,100 feet).
"Sunlight doesn't get more than about 300 or 400 metres into the sea and we're talking about 4,000 metres depth... literally working in the dark," Dolan said.
"We've got the capacity to take video images and photographs of areas, but we will only turn that on when we think we've got something that needs a closer look."
The challenges reflect how little is known about the world's open oceans.
"We know more of the moon than we know about the bottom of our oceans. The maps that we have of the moon are 25 times better than the maps we have of our oceans," oceanographer Erik van Sebille, of Imperial College London and an adjunct lecturer at Australia's University of New South Wales, told AFP.
The mapping of the ocean floor in the MH370 zone and other areas could eventually yield insights into sea life, habitat, and even mining potential, he said.
So far, 2,000-metre cliffs, volcano clusters, undersea mountains, ridges and valleys -- similar to land features -- have been discovered in the area MH370 is thought to have come down, Dolan said.
Helping future searches
The probing of such a vast and remote area means officials have broken new ground while trying to ensure reliable data is recorded, the search chief added.
A radar on an Indonesian National Search and Rescue boat shows details during a search in the Andaman sea area for the missing Malaysian Airlines flight MH370 on March 17, 2014.
"We've probably taken a step forward there (on the) quality, including the quality assurance of the data," Dolan said.
"The lessons are... that the planning and coordination of this is complex and requires a lot of effort.
"We drew extensively on the experience of our French colleagues with Air France 447, so we learnt from them. We'll be able to pass on (our lessons) to others who may come after us."
The MH370 probe has also helped prove the effectiveness of the next generation of search tools, including synthetic aperture sonar -- which collects higher-resolution readings.
Dolan remains confident the more than 200-strong international team working to find the jet will be successful, but cautioned: "Because of the nature of the (satellite data) calculations, there's no absolute guarantee it's there, just very likely."
Van Sebille likens the search to looking for a set of house keys lost in London in complete darkness.
"That's the kind of scale that you have to work with. But of course if you don't try at all, you are certain not to find it," he said.