Swashbuckling adventurer Steve Fossett vanished last September after taking off in a borrowed plane from a private airstrip in western Nevada.

In a quest to find the multimillionaire and his small plane, authorities and volunteers clocked more than 13,000 hours in the air, scoured some 20,000 square miles -- including a chunk of the Sierra Nevada -- and encouraged armchair detectives to study online satellite imagery.

Yet Fossett's disappearance remains a mystery one year later.

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 "wreckchasers" -- 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," said Craig Fuller of Aviation Archaeological Investigation and Research, a Web-based source of military accident reports and other aviation archaeology information. "But on average it takes four to six trips."

Concealed by terrain

The 400-mile-long Sierra is a boneyard of aircraft wrecks.

The Air Force Rescue Coordination Center in Tyndall, Fla., has mapped nearly 180 crashes within the mountain range -- mainly so searchers can distinguish older wrecks from new ones. Wreckage often is left behind in rugged terrain because it's too tough to haul out.

Many wrecks -- but not all -- have been visited by rescuers or authorities and then forgotten again, once bodies were recovered. No one knows for sure how many other wrecks sit undiscovered.

Fossett's disappearance offers insight into the mystery of missing aircraft.

About 9 a.m. on Sept. 3, 2007, Fossett left a remote ranch near Yerington, Nev., 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 was headed toward Bishop in the eastern Sierra, an area known for huge gusts of wind in the fall.

Fossett, 63, built a fortune by trading futures and options on Chicago markets. He frequently risked his life in pursuit of aviation records and adventure.

In 2002, he became the first person to circle the world on a solo balloon flight. He also swam the English Channel, finished the Iditarod sled-dog race and climbed several world-famous mountain peaks.

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 coordination center suspended the main search by the Civil Air Patrol -- which coordinated with other agencies. The air patrol is a nonprofit and all-volunteer auxiliary of the Air Force.

The failure to find Fossett "is a testament to the unforgiving terrain comprising the search area," Lt. Col. 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."