The clues are tantalising. There are parts of aircraft skin, an apparent camp site, a woman's cosmetic jar for freckle ointment from the 1930s and a zip made in Pennsylvania from the same era. But on nine previous expeditions, Tighar has failed to prove conclusively that the remote atoll was where Earhart died as she tried to circumnavigate the globe.

The evidence that would win over the sceptics who prefer to stick to the theory that she was a spy abducted by the Japanese, or that her Lockheed Model 10E Special 'Electra' simply ran out of fuel as they searched in vain for the airfield on Howland Island, would either be in the form of a bone that can be identified through DNA or an aircraft part that is without doubt from the Electra.

And on this trip, which sets sail from Hawaii on July 2, 75 years to the day from when Earhart made her last radio transmission as she searched for Howland, Tighar is better equipped than any previous efforts to test its theory.

Gillespie says there is evidence to suggest the aircraft was landed on the reef flats at the western end of the island but later washed over the reef and down the steeply sloping flanks of the sea mount. Previous expeditions have been defeated by the depth of the ocean, but this trip has earned the backing of the United States government and will involve a multi-beam sonar system creating a detailed map of the crags and canyons that make up the reef slope. Any potential targets will then be examined more closely by a remotely operated vehicle fitted with a camera.

And while much of the aircraft is likely to have been scattered in the intervening years, Tighar believes that some key components, such as the Pratt & Whitney engines, could still be where they sank 75 years ago.

Asked what the best result from the 26-day expedition would be, Gillespie summed it up as, 'photos of conclusively identifiable wreckage from the Electra aircraft'.

'We have good evidence of what happened up to the time the plane went over the reef edge and into the water, but there are many things that could have then happened that would make any wreckage unfindable,' Gillespie says.

'If we don't find anything we'll be disappointed, but not discouraged.'

But the physical proof may be just waiting to be discovered.

'Any man-made objects found will be photographed and their location carefully recorded,' Tighar said on its web site. 'No recovery of objects will be attempted unless necessary to confirm identification.

'Should identifiable wreckage from the Electra be discovered it will be documented as thoroughly as possible in situ so that a separate expedition can be equipped with the appropriate means to recover and conserve the materials.'

The expedition is being filmed by the Discovery Channel for a television documentary and has aroused sufficient interest in Washington for the White House to give it the official seal of approval, in significant part due to new forensic examination of a grainy, black-and-white photo taken three months after Earhart disappeared that appears to show part of an aircraft on the reef.

Announcing in March that the government would back the search, Hillary Rodham Clinton, the US Secretary of State, said: 'We can be as optimistic and even audacious as Amelia Earhart.'

That support does not come with funding, however, and Tighar is nearing its US$2 million target to cover the costs of the expedition.