Officials and volunteers are searching for American adventurer Steve Fossett, who went missing Monday while piloting a small plane.
Fossett, 63, disappeared after taking off in a remote area in the western state of Nevada.
Fossett was scouting the rugged terrain for dry lake beds where he could attempt to break the world's land speed record. He was expected to return within a couple of hours.
Authorities say he did not file a flight plan, and was not required to do so.
His plane had an emergency locator radio beacon, but rescuers have not picked up any signals.
Fossett, who swam the English Channel, is well-known for his record-setting adventures in balloons, sailboats, gliders and powered aircraft.
In 2002, he became the first person to fly solo around the world in a balloon, and in 2005, set the record as the first person to fly an airplane solo around the world without refueling.
Richard Branson, the British entrepreneur who sponsored Fossett's 2005 flight, says the American "is a tough old boot" who is waiting for someone to rescue him.
Some information for this report was provided by AP.