Have you ever flown over the continental United States, looked out the window, and saw giant concrete arrows in the landscape? If you have, you probably scratched your head and wondered just why they were there. These concrete arrows were installed in the 1920s to help pilots navigate when it became too dark. Remember: Back in the 20s, there was no GPS, even on planes, to help guide them to and fro.

This concrete arrow system was part of the now defunct Transcontinental Airway System, originally created for air-mail pilots. During the 1930s, this system utilized more than 1500 beacons over roughly 18,000 miles. Each arrow was made of concrete and painted bright yellow. In the center, it had a steel tower that was topped by a candle-powered rotating beacon. Below that, there would be two sets of lights going forward and backward on the arrow. These lights would flash codes, identifying each beacon so that pilots would know where they were and could adjust course if necessary.

Previously, many of these beacons were set alight by fire, giant bon fires that had people managing them. As you can imagine, this could lead to more navigational errors than they could solve. These bonfires were also used on taxiways and landing strips, giving pilots a means to see where they were landing. By the 1940s, as radar and GPS technology was in its infancy for air travel, these beacons and bon fires began to fall to the wayside.

However, as they fell to the wayside, many of the concrete arrows and beacons were taken down, recycled or just disposed of in landfills. Some of these concrete arrows remain and can be seen by pilots, and those passengers, as they transverse the United States. The only area of the country that still actually uses these concrete beacons is Montana, where they make navigating mountainous terrain easier for pilots, especially small planes.