In 1924, air mail pilots were having trouble finding their way across the featureless American southwest, so the Post Office adopted a brutally low-tech solution: Every 10 miles they built a large concrete arrow illuminated by a beacon. Each arrow pointed the way to the next, so that a pilot could stay on course simply by connecting the dots.

The system was finished by 1929, permitting mail planes to find their way all the way to San Francisco. It was quickly superseded by more sophisticated navigation methods, but today the arrows still dot the American desert, ready to confuse hikers and, probably, future archaeologists.

(Thanks, Ron.)