Legends about plunging ice go back for centuries. They didn't begin to receive serious scientific attention until a few years ago, however, when Spain and other countries were pelted by the mystery intruders.

Possible explanations range from the mundane to the bizarre.

One theory is that ice is somehow forming on the outside of aircraft, perhaps in areas that aren't protected by deicing equipment, said David Travis, a climatologist at the University of Wisconsin at Whitewater. Last year, he and 11 others co-wrote an article on the ice-fall mystery in the Journal of Atmospheric Chemistry.

Lead author Jesus Martinez-Frias of the Planetary Geology Laboratory in Madrid and his colleagues have collected reports of 40 cases around the world since 1999 of puzzling falling ice, or "megacryometeors," as they call the strange objects.

Martinez-Frias hypothesizes that the ice forms in the upper atmosphere by a process similar to the formation of hail inside thunderstorms but without a thunderstorm.

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