KEY POINTS:
Adventurer Steve Fossett vanished last September after taking off in a borrowed plane from a private airstrip in western Nevada.
Over the past year, in a quest to find the multimillionaire and his small plane, authorities and volunteers have clocked more than 13,000 hours in the air, scoured more than 32,000 square kilometres _ including a chunk of the Sierra Nevada mountain range _ and encouraged armchair detectives to study online satellite imagery.
Yet Fossett's disappearance remains a mystery.
How could this iconic aviator plummet from the sky without a trace?
How could one of the largest and most intensive searches in modern history fail to yield results?
The answer, experts say, is that a plane wreck is one of the hardest needles to find in a rugged haystack _ especially the Sierra.
Pilots don't always file flight plans.
Emergency radio beacons may not activate in a crash.
Airplanes can slide under trees or bushes, slip into lakes, scatter into bits or be buried by snow.
Even experienced "wreck-chasers" _ a growing group of hobbyists _ can be thwarted while hunting for an already documented site.
"Sometimes we'll go out and find the site on the first try," Craig Fuller of Aviation Archaeological Investigation and Research, a web-based source of military accident reports and other aviation archaeology information, told the Fresno Bee. "But on average it takes four to six trips."
The 645km-long Sierra is a boneyard of aircraft wrecks.
The Air Force Rescue Co-ordination Centre in Tyndall, Florida, has mapped nearly 180 crashes within the mountain range _ mainly so searchers can distinguish older wrecks from new ones.
Wreckage is often left behind in rugged terrain because it is too tough to haul out.
Many wrecks _ but not all _ have been visited by rescuers or authorities and then forgotten again, once bodies have been recovered.
No one knows for sure how many other wrecks sit undiscovered.
Fossett's disappearance offers insight into the mystery of missing aircraft.
About 9am on September 3, 2007, Fossett, 63, left a remote ranch near Yerington, Nevada, owned by hotel magnate William Barron Hilton, in a single-engine Citabria Super Decathlon.
He didn't file a flight plan. Published news reports later said Fossett, who in 2002 became the first person to circle the world on a solo balloon flight, was headed towards Bishop in the eastern Sierra, an area known for huge gusts of wind in autumn.
Last year, agencies and volunteers from around the country invested thousands of hours in the search.
Experts reviewed radar data. Crews in the air used cutting-edge technology to search the ground.
The public helped in the hunt by examining satellite photos over the internet.
After a month, the rescue co-ordination centre suspended the main search by the Civil Air Patrol, which co-ordinated with other agencies.
The failure to find Fossett "is a testament to the unforgiving terrain comprising the search area", Lieutenant Colonel E.J. Smith, the patrol's search leader, said last October.
"The combination of high altitude, thick forest and mountainous terrain proved to be unconquerable during this particular search operation."
A judge declared Fossett dead in February.
Private searches are continuing. Pat Macha, an aviation archaeologist who has investigated historic plane wrecks, said the search window is narrowing if Fossett crashed at high elevation.
Days are becoming shorter and winter increases the possibility that snow could bury the plane.
"If they don't find him by October 1 this year, it's going to be a long, long time," Macha said.
Those involved in aviation archaeology and wreck-chasing _ or wreck-finding _ often are lured by the mystery and thrill of this most difficult of searches. The hobby often requires hiking, climbing and detective work.
Longtime wreck-chasers say they want to explore, document and preserve historic crash sites as memorials to people who suffered or died there.
Macha, co-author of Aircraft Wrecks in the Mountains and Deserts of California, has tramped across all kinds of terrain for more than 40 years.
Macha was just a high school student in 1963 when he stumbled upon the wreck of a Douglas C-47B in the San Bernardino Mountains.
A hand-painted, yellow "X" marked on the plane signified it wasn't a new discovery or a new wreck.
The military transport hadn't burned.
It was surrounded by parachutes and other personal effects from the 13 men who died there in 1952.
Their bodies had been removed.
Macha, who lives in Southern California, has since hiked to hundreds of historic crash sites _ dodging bears, snakes and more.
The retired history and geography teacher has helped identify wrecks as well as connect relatives of fallen aviators with pieces of their past.
Fuller, who has collected thousands of military accident reports, is devoted to the field.
He has a bachelor's degree in aeronautical science and teaches occasional classes on how to find, research and document historic crash sites.
"The more remote the site, usually the more that's left there," Fuller said.
"It's exciting to see big pieces but I'm just as excited at a site where everything will fit into a 5 gallon paint can."
Someone may locate Fossett's crash, he said.
But if the plane had burned, it may look like nothing more than a pile of twigs or even a bush from the air.
"It's going to be a hiker or a hunter who's going off the beaten path that finds it, most likely," Fuller said.
- NZPA
