It was an unusual combination of weather and a supersonic Air Force jet that sent a mysterious rumble along the Wasatch Front Thursday afternoon.

A radar-jamming EF-111 was flying at 45,000 feet over the Salt Lake area, pushing sound waves along as it sped toward Mountain Home Air Force Base in Idaho after flying a training mission to Nellis Air Force Base near Las Vegas."It's not at all unusual for the EF to be traveling that fast," said Christine Catlett, public affairs officer at Mountain Home. "The cloud cover (in Salt Lake) created the chain reaction of booms."

Air Force jets fly hundreds of missions high above the Salt Lake valley, she said. "If the skies had been clear nobody would have heard anything." The sonic booms just aren't heard on the ground from high-flying aircraft like the EF-111, an aircraft similar in appearance to the F-111 fighter, she said. Even when the skies are clear you would have a hard time seeing one of the aircraft at that altitude, let alone hearing it, she said.

The noise took several minutes to travel the length of Salt Lake Valley. It was felt in places as far apart as Lehi and Salt Lake City, and registered on seismographs at the University of Utah.

Curiosity about the source of the boom set the phones ringing at newspapers, radio stations and police departments in the Salt Lake area. Many calls also went to Hill Air Force Base because of suspicions the concussion had come from a supersonic airplane.

"We don't know exactly where it was, but we heard the boom here at Hill," said base public affairs officer Sylvia Le Mons-Liddle. Hill personnel knew the boom hadn't come from one of their aircraft so they set about finding out where it had come from - partially to satisfy their curiosity, but mostly to answer the deluge of calls.

Utahns who heard the mysterious boom may have feared a planned high-explosive test blast detonated prematurely in the western desert. But it hadn't. The detonation, one of the largest in Utah history, is still set for Monday, provided the weather is good. That will be the Air Force's test detonation of a 500,000 pounds of high-explosives in the West Desert to test the structural soundness of a new storage unit called a Hayman Igloo. The blast will be held in the Air Force's Utah Test and Training Range west of the Great Salt Lake.

The boom will be twice as large as the range's previous record explosion, a test conducted 20 years ago.

Officials of the Ogden Air Logistics Center wrote an environmental impact statement that concluded there would be no significant impact to the environment.

"The test is designed to help develop safe storage of air munitions to meet Air Force requirements," said Sylvia LeMons-Liddle of Hill AFB.

The test will involve a new, highly economical storage structure called the Hayman Igloo. Two of the special igloos will be constructed and the high explosives will be detonated in one of them. The other unit, a short distance away, will be loaded with instruments to measure the blast's effects.