One of aviation's most enduring mysteries remains unsolved after an object thought to be the wreckage of famed US aviator Amelia Earhart's plane turned out to be washed-up trees.
US-based explorers set out on a $900,000 expedition to Kiribati in the South Pacific this month seeking to conclusively solve one of aviation's most enduring mysteries.
Earhart was hoping to become the first woman to fly around the world before her 1937 disappearance.
Amelia Earhart at the Lockheed Electra she attempted to fly around the world. (Supplied)
Researchers hoped to learn whether a multi-pronged object detected by 2020 satellite imagery in a lagoon on one of Kiribati's tiny islands was a remnant of her long-lost Lockheed Electra 10-E airplane.
The finding would have decisively proven theories that Earhart and her navigator Fred Noonan crash-landed and perished on or near the coral-ringed atoll.
'Adventure is worthwhile'
Backed by private donors and Indiana's Purdue University, the team spent a week on the island before concluding the "visual anomaly" picked up by satellite "was likely a collection of large trees", an expedition statement said.
"Unfortunately, we weren't able to find that smoking gun," Purdue researcher Steve Schultz said in a video message from the Kiribati island of Nikumaroro.
Quoting Earhart, he added: "You know, Amelia herself said, 'Adventure is worthwhile in itself.'"
Much of the debris had fragmented and washed away, leaving a smaller Y-shaped segment.
Its "carbonate veneer" produced a reflective surface suggestive of metal, the team said in a statement.
Sonar vision of what researchers thought could be part of Amelia Earhart's plane wreckage. (Supplied: Deep Sea Vision)
Two previous attempts to locate the wreckage in the ocean around Nikumaroro also proved fruitless, while a plane-shaped sonar image captured elsewhere on the sea floor turned out to be a pile of rocks.
The Electra aircraft was last seen taking off from Papua New Guinea on July 2, 1937.
Radio contact was lost the next day after Earhart reported "gas is running low", followed by her final known transmission giving her navigational position.
Castaway hypothesis 'remains very strong'
The disappearance spawned one of the largest air-sea searches ever undertaken and remains one of the great unsolved mysteries of the 20th century.
Expedition leader Richard Pettigrew, head of the Archaeological Legacy Institute of Eugene, said evidence supporting the "castaway hypothesis", suggesting Earhart and Noonan died while marooned on Nikumaroro, remained "very strong".
American aviatrix Amelia Earhart poses with flowers as she arrives in Southampton after her transatlantic flight on the Friendship from Wales in 1928. (AP)
He cited evidence collected by veteran Earhart explorer Ric Gillespie, including multiple distress calls picked up in the days after she vanished with bearings that converge on Nikumaroro, and a 2018 forensic analysis of human bones recovered on the island in 1940 showing they belonged to a woman of Earhart's build and stature.
Mr Gillespie's investigation also encompassed artefacts dating to the 1930s found on the island such as a woman's shoe, a compact case, a freckle cream jar and medicine vial, as well as remnants of a jackknife like one Earhart carried on the plane.
An object appearing to be the Electra's landing gear can also be seen in a photograph of the island's shoreline taken months after Earhart vanished.
A number of other artefacts were uncovered by this month's Purdue expedition, including a 76-centimetre-long steel tube found in the lagoon.
Mr Gillespie believes the plane has been lost to history, eventually swept by tides and carried into the sea to sink and break apart in the surf.
The US government's official view has long been that the plane ran out of fuel and crashed at sea near its refuelling destination of Howland Island, about 640 kilometres north-west of Nikumaroro.
AP