One group, led by Simon Donato, a Canadian geologist and adventure racer, began a weeklong effort on Monday to solve the mystery surrounding Mr. Fossett, who took off from a ranch in the Reno area on Sept. 3 for a brief jaunt in a single-engine plane and never returned.

One of the most extensive, complex and costly searches for a missing aircraft in American history failed to find him last year. In February, at the request of his widow, Peggy, an Illinois judge declared him legally dead.

Mr. Donato, who could not be reached for comment Monday, told The Associated Press over the weekend that his 10-member team included paramedics and expert hikers who would conduct a "blitzkrieg" search of remote canyons and forested regions inaccessible to most.

Should that effort prove unsuccessful, another adventurer, Robert Hyman, intends to lead a team in August on a similar volunteer search. His group, made up of experts in aviation, mountain terrain, hiking and climbing, has set no limit on the length of its quest.

Mr. Hyman, a Washington investor who has led a variety of scientific expeditions at Mount Everest and throughout Central America, said finding Mr. Fossett could be crucial to understanding why the search-and-rescue efforts of last year failed.

"They really brought it all out, so why didn't they find him?" Mr. Hyman said in an interview on Monday. "The answer must be that he can't be seen from the air. What's blocking the eyes in the sky? He's probably under rocks, under a ledge. We just want to find Steve, and we want to help with the methodology with search and rescue."

Neither hunt is sanctioned by Peggy Fossett, who helped finance a large private search last fall in addition to one conducted by the Civil Air Patrol and various Nevada and California agencies. Mrs. Fossett has given no interviews since her husband's disappearance but issued a statement recently in which she said she had "no further plans for additional searching."

Both Mr. Donato and Mr. Hyman are members of the Explorers Club, an international organization of adventure enthusiasts to which the 63-year-old Mr. Fossett al so belonged.

Mr. Fossett was famed for his dozens of sea, land and air records, including a nonstop solo trip around the world in a balloon. His status, and the unusual circumstances surrounding his presumed death, have piqued interest similar to that concerning the fate of Amelia Earhart, who vanished over the Pacific on a round-the-world flight in 1937 and was never found.

"There is something of a parallel there: it's simply a mystery," said the Explorers Club spokesman, Jeff Stolzer, who noted that members of the club were still involved in Earhart searches.

"Steve was a beloved member of the club," Mr. Stolzer said, "and an amazingly nice man. People were really saddened when he disappeared. They feel like it's almost an obligation to find out what happened."