Federal prosecutors formally dropped charges this month against an amateur astronomer who exposed a buried surveillance network surrounding the Air Force's mysterious "Area 51" air base in Nevada.

Chuck Clark, 58, was charged in 2003 with a single count of malicious interference with a communications system used for the national defense, after prosecutors held him responsible for the disappearance of one of the wireless motion sensors buried beneath the desert land surrounding the facility.

In a deal with the government last January, Clark agreed to enter a one-year term of "pretrial diversion" -- a kind of probation -- and to either locate and return the lost device, or make financial restitution to the Air Force. "He paid for the missing sensor, and complied with the conditions of his pretrial diversion and the case was dismissed," says Natalie Collins, a spokesperson for the U.S. Attorney's Office in Las Vegas.