Dozens of airplanes and helicopters took off at dawn Monday to crisscross the Nevada high desert on the eighth day of the search for missing aviator and adventurer Steve Fossett.

There hasn't been a trace of Fossett or the single-engine airplane he was piloting since he took off from an airstrip at an isolated ranch Sept. 3.

Hopes were suddenly raised — but just as quickly dashed — Sunday afternoon with a false sighting, adding to the growing frustration felt as search parties have spotted wreckage of eight other airplanes that had been lost for years in and around the rugged mountains of western Nevada.

"This search is big, it is frustrating and it is exhausting, physically and mentally," Civil Air Patrol Maj. Cynthia Ryan said Sunday.

Click here to read more about the rugged terrain.

"What's so frustrating is we haven't found anything," added Lyon County Undersheriff Joe Sanford.

The search covers 17,000 square miles — twice the size of New Jersey — over mostly Nevada wilderness and parts of California on the eastern front of the Sierra Nevada range.

On Sunday, part of the search was concentrated on the area within a 50-mile radius of the airstrip Fossett where took off on Labor Day about 80 miles southeast of Reno. Most crashes occur within that radius of an airport, Ryan said.

For a moment, the change in strategy appeared to pay off as leaders of the operation told reporters at Minden-Tahoe Airport that a helicopter and ground crew were en route to a possible sighting.

But a half-hour later, the ground crew arrived at the scene 30 to 40 miles east of the south end of Walker Lake and confirmed the wreckage was not Fossett's plane.

The debris was from an old Navy crash, said Kim Toulouse, a spokesman for the Nevada Department of Wildlife. He had no other details.

Later Sunday, search crews discovered yet another old crash site but could tell immediately it was not Fossett's plane, he said.

"We're finding them left and right. Nevada is a graveyard," Toulouse said. "The technology we have today is allowing us to find this stuff."

It was at least the eighth time since Fossett disappeared that search crews thought they had located him only to learn the wreckage came from crashes that were years or even decades old.

Fossett's wife, Peggy, remained Sunday at hotel magnate Barron Hilton's ranch, where Fossett took off for what was to be a three-hour trip.

Sanford said Sunday he had spoken to members of Fossett's family.

"The mood is very somber but very focussed," Sanford said. "I can't imagine being on the receiving end of hope that continues to turn out to be unfounded or be something we were not aware of."

Fossett, a former commodities trader who was the first to circle the globe in a balloon, is considered an expert pilot and survivalist.

The discovery of the previously unknown wrecks suggests how difficult it might be to find Fossett's single-engine Bellanca Citabria Super Decathlon.

The Florida-based Air Force Rescue Coordination Center, which is helping coordinate the search, maintains a registry of known plane wreck sites.

The registry has 129 entries for Nevada. But over the last 50 years, aviation officials estimate, more than 150 small planes have disappeared in the state, which has more than 300 mountain ranges carved with steep ravines and covered with sagebrush and pinon pine trees. Peaks rise to 11,000 feet.

"The mountains are quite rugged, and things don't always get found," Ryan said.