On Dec. 15, 1989, KLM flight 867 from Amsterdam was approaching its destination in Anchorage, Alaska, when the plane flew into what appeared to be a thin layer of normal clouds. Suddenly, according to flight-crew reports, it got very dark outside and the air in the cockpit filled with a brownish dust and the unmistakable smell of sulfur. One minute after beginning a high-power climb to escape...
"Let me start off with a riddle," says NASA scientist Allan J. Zuckerwar. In his office in Hampton, Va., he rattles off items as dissimilar as rhinoceroses, supersonic aircraft, and hurricanes. "Now, what do they have in common?" The answer, Zuckerwar explains, is that each one generates silent infrasound—long sound waves at a frequency below 20 hertz. People can't hear anything below that...