Take to the heavens! It seems like a good idea. Dodge all the obstacles of the terrestrial rat race, in the limitless, empty, peaceful skies. But it's not as empty up there as it looks. Icarus may not have had to worry about being sucked into the engine of a jet, but even he would have faced, for example, a cloud of migrating bugs once he reached 1,000 feet above sea level. And there's more weirdness where that came from. Satellite images, radar tracking, and old- fashioned air samples show a sky teeming with machines and life. So next time people describe something as "heavenly," you might ask them if they mean "crowded."

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