In short:
A team has completed a fresh search to find the remains of 20 American servicemen killed when a military plane was shot down off the Western Australian coast in 1942.
It is part of ongoing efforts to recover the remains of 80,000 American service personnel missing globally, and locate thousands of wrecks undiscovered in Australian waters.
What's next?
Volunteer wreck-hunters are planning a fresh search for the warplane next month in waters near Broome.
The US government has launched a fresh mission to find an American aeroplane missing in Australia's northern waters, more than 80 years after it crashed, killing more than a dozen men.
It is the latest in a series of expeditions aimed at finding the thousands of shipwrecks and sunken planes that lie unrecorded in Australian waters — many containing human remains.
The latest search was funded by the US Department of Defense, in a bid to recover the remains of soldiers killed when a US Liberator plane was shot down by Japanese fighter planes near Broome in 1942.
"It's a bit of needle in a haystack kind of scenario," says maritime archaeologist Jane Mitchell, who led the expedition.
"We didn't find it this time, but we've been able to rule out an area of about 15 square kilometres, so in that sense, no result is still a result.
"[The crash] is still in living memory and there are families wanting to know what happened to their loved one, so it's an honour to have the opportunity to try to answer some of those questions."
Thousands of wrecks
More than 11,000 boat and plane wrecks lie in Australian waters, but only about one-quarter of them have been located.
Some are boats sunk hundreds of years ago, such as Makassan and Dutch vessels.
Others are aeroplanes shot down in wartime, navy boats sunk in battle, or luxury liners such as the SS Koombana, which sank, killing 150 people. It has never been found.
With search technology becoming cheaper and more accessible, authorities are expecting a spike in discoveries in the coming years.
"We've really seen in the last 10 years how technology has come along, and suddenly it's making these sites much more accessible," says maritime archaeologist Ross Anderson.
"Along with increased seabed mapping, I think we're going to see a peak and a rise in new discoveries."
A global search
The most recent search for the Liberator was commissioned by the US Defense POW/MIA Accounting Agency, which oversees a database of more than 80,000 defence personnel whose bodies have not been recovered.
The agency's historian Dan Kosasa says about 280 of those are believed to have died in Australian territory.
"Recovering the remains of our missing personnel and returning them to their families is our agency's mission, and we go to exceptional lengths to accomplish this mission," he says.
Even after 80 years, the unit is getting tip-offs from Australia.
"Just recently, the agency learnt of a lead in which some personnel working at a navy yard in Australia during the war may have discovered American remains in the wreckage of a vessel they were repairing and buried them without notifying the US government," Mr Kosasa says.
In search of 'the Holy Grail'
Helping solve the maritime mysteries are amateur enthusiasts such as Jim Miles.
The retired military pilot has been searching for the Broome Liberator wreck for 15 years, towing a side-scan sonar from his catamaran to scour the sea floor.
The small device emits pulses reaching 50 metres that are fed back to create a digital image of every bump and object.
"It's hard to believe that all around Australia there are still these missing wrecks," he says.
"Finding them is a bit of drug, to be honest.
"And finding this Liberator is, for me, the Holy Grail."
The aircraft was shot down on a steamy wet-season morning in March 1942, just seven minutes after take-off.
It was full of injured and sick American servicemen being evacuated from Java to Australia, and while the records of those aboard are not precise, it is believed at least 20 men died.
Miraculously, a young soldier from Oklahoma survived and swam for 33 hours to reach shore. Two days after the crash, Sergeant Melvin Donoho staggered into the Broome airbase, naked, sunburnt and close to collapse.
The circumstances of the crash create a tantalisingly limited search zone stretching about 16 kilometres offshore.
'A spiritual moment'
When the ABC joins Mr Miles aboard his search boat, he is moored in the turquoise waters off Broome, marking targets on a nautical map.
"I've got a tip from local fishermen about some possible structure underwater, so we'll head out there and check it out," he says.
"Most days you don't find anything, but you never know — today could be the day."
Pinpointing an underwater grave requires historical detective work, involving flight manifests, black-and-white photos, tidal charts and witness testimony.
In 2014, he and long-time search partner Dion Marinis found the wreck of an American Beaufighter also shot down during the Japanese attack on Broome.
They were able to meet with relatives of the men killed in an emotional memorial ceremony a few months later.
"When you find a wreck and descend down knowing no-one's seen it since 1942 or whenever … it's a really spiritual moment," Mr Miles says.
"It's always on your mind that you're looking for a site where potentially a lot of people died, and there are families waiting to learn of a final resting place."
Ross Anderson, curator of maritime heritage at the WA Maritime Museum, says volunteers play a vital role in documenting Australia's underwater sites.
"There are quite a few people who may not be professionally qualified archaeologists, but they're trained up and passionate about finding and documenting sites," Dr Anderson says.
"Not all searches are successful, so that's a real testament to people like Jim who keep heading out there to keep the search alive."
'Leave no-one behind'
It's likely human remains could still be recovered from the Liberator wreckage once it's found.
"It's a sensitive subject, but human remains that are buried in the seabed can be well-preserved," says Dr Anderson, who has taken part in previous searches for the Liberator.
"It's always a paramount thing to locate personnel, and lay them to rest and respect their sacrifice."
Just this year, the remains of an American serviceman were retrieved from a wreck located in Papua New Guinea and buried in the United States in a solemn ceremony.
It's expected the number of wreck discoveries will increase in the coming years amid an international push to map the entire global sea floor by 2030.
According to Geoscience Australia, 39 per cent of Australia's 14 million square kilometres of ocean floor has been mapped, with the percentage increasing each year.
The recent searches for missing Malaysian Airlines flight MH 370 resulted in the incidental discovery of two previously unrecorded shipwrecks.
But the cost and danger involved in finding deep-sea sites remains prohibitive.
Dangerous deep dives
The Perth-based Wrecksplorer team has succeeded in finding more than a dozen previously unrecorded wrecks, clustered around Wadjemup/Rottnest Island.
Alex Aberle-Leeming says the team of six technical divers are all volunteers, scheduling expeditions around demanding day jobs.
"Our specialty is deeper diving, where the majority of wrecks haven't been dived," he says.
"It takes a lot of planning, but it's a great privilege to be down there and share what we've found."
Once they have descended to a wreck, the team uses high-powered video lights and cameras to snap two images per second to create a three-dimensional recreation.
"There is so much out there to be found, and none of us are interested in being paid; we're doing it for the love of it," he says.
Final resting place
For now, the Liberator and its final passengers lay entombed on the ocean floor, likely within 20km of the bustling town of Broome.
According to US Defense, the search for those who died will continue for as long as it takes.
Its missing-in-action agency meets monthly with the Australian Defence Force's Historic Unrecovered War Casualties units to compare notes and discuss fresh evidence.
"We see ourselves as carrying on the legacy of our two nations' close collaboration during WWII," reflects military historian Dan Kosasa.
"The process is not simple or fast.
"But if it is technologically possible … and we believe there is the possibility, even if somewhat remote, that we recover remains from the wreckage, then we will do so."
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