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Why Is America Dotted with Giant, Concrete Arrows?

These old air-mail beacons are visible all over the land (if you know where to look).
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Back in the 1920s, a pilot lost on a dark, thunderous night couldn't depend on GPS to save his bacon. But there was something almost as good: giant, cartoon-style arrows, stretching in an illuminated path on the ground from New York to San Francisco.

This rudimentary form of navigation was part of the Transcontinental Airway System, an effective aid for air-mail pilots that by the '30s incorporated 1,500 ground beacons over some 18,000 miles. NASA ambassador Patrick Wiggins recently explained how it worked for the Universities Space Research Association: