If you’re wandering the barren landscape of Utah or Nevada and stumble across a massive, firmly-grounded concrete arrow, rest assured, aliens have not been marking a path through the United States. Forming a line across the country, these decaying markers are actually remnants of the Transcontinental Air Mail Route, a revolutionary system connecting both coasts over 90 years ago to facilitate speedy mail delivery. But given that the full path is only visible from an aerial view, like the Peruvian Nazca lines, their strange appearance has puzzled many hikers and ranchers who have stumbled across a lone, giant arrow.

Before 1920, keeping in touch over long distances was a very slow affair. Mail often took weeks to get from one coast to the other, chugging along on rail lines. The Pony Express stopped galloping in the mid-1800s, and the train system was booming. But as the Roaring Twenties approached, a new, faster method of parcel delivery was burgeoning: airmail. The year 1918 marked the first time mail was delivered by air, only available for short distances at first. In 1920, the United States Postal Service announced it would begin a coast-to-coast route. And in 1921, a team of seven pilots completed the first harrowing delivery, proving to a skeptical Congress that airmail was possible and that the program deserved funding.

From then on, brave pilots in 96 propeller planes flew correspondence back and forth across the country, cutting those weeks needed to deliver a letter by train down to a mere 34 hours. The only problem? Radar, radio systems, and aviation maps didn’t exist. Pilots were left to navigate the U.S. using landmarks, and were grounded after nightfall and during bad weather. The era may have been thriving for on-the-ground transportation, but in the 1920s, the skies were still uncharted territory and pilots were left nearly blind.