(CNN) — Picture the scene: you're driving through the arid American desert. There's been nothing around for miles. Suddenly, up ahead, you spot a gigantic concrete arrow.
No, you're not hallucinating as you travel. These enigmatic structures are dotted across the length and breadth of the United States -- they were built in the late 1920s and early 1930s to help guide pilots navigating the country's fledgling air mail system in an era long before satellites and GPS. Back in the day, the arrows were illuminated by neighboring beacons. Now, most of these light towers are long gone and the arrows lie abandoned -- some visible on Google's aerial maps. For those in the know, tracking these strange formations can be a compelling pastime. Retired Californian couple Brian and Charlotte Smith drive around the United States, snapping photographs of the arrows for their specialist website Arrows Across America.
These mysterious arrows are dotted across America. Pictured here: Arrow in Nevada on the Los Angeles-Salt Lake airway.
The Smiths discovered the indicators almost by accident after Brian, a former police officer with the California Highway Patrol, was sent an email about them. The couple was immediately intrigued.
"Charlotte got really interested in it and wanted to find what it was, so started researching, looking for them on the internet," Brian tells CNN Travel.
She found little to go on.
"What sparks my attraction to the arrows was that this existed and I had no clue about it, and there was no information about it," says Charlotte, a former genealogy researcher.
She's right, there's really not much information available online. Contrary to popular belief, the arrows have no association with the US Post Office.
The arrows were illuminated by neighboring beacons. Most of them are long gone, but a few survive -- including this one in Carbon County, Wyoming on the Salt Lake-Omaha airway.
Jenny Lynch, historian and corporate information services manager at the Postal Service and Nancy Pope, curator at the Smithsonian's National Postal Museum, separately tell CNN Travel the arrows were built when the Department of Commerce took over the airways.
The arrows were built between 1926-1931, according to the Journal of Air Traffic Control, produced by the Air Traffic Control Association. Beacons were 25 miles apart from one another and the arrows were painted yellow -- although the paint on most of the remaining indicators has now faded.
Today, the Smiths are concerned they're now being painted quirky colors.
"People paint these things bright orange and it ruins them really," says Brian.
The arrows tend to be in isolated locations. Pictured here: Arrow in Sweetwater County, Wyoming on the Salt Lake City-Omaha Airway.
"When I found an arrow, I marked [the location], and started making a spreadsheet," explains Charlotte.
All the arrows are in fairly isolated locales -- the first arrow the Smiths visited was no exception.
"It was in Nevada, right by Reno," says Charlotte. "Brian ended up having to climb up the hill as I couldn't climb it at all, I've got really bad knees. We took a lot of pictures."
Despite the difficulties, they were hooked -- and Arrows Across America was born.
The beacons were officially decommissioned in the 1970s. Pictured here: Arrow in Siskiyou County, California, adjacent to Montague Airport on the San Francisco-Seattle airway and San Francisco-Redding section.
The couple host a comprehensive website featuring photographs, coordinates and information on each of the arrows they've found.
Brian, 70, and Charlotte, 67, have been photography buffs for a while now. "When Brian retired, we took a Photoshop class," says Charlotte. "We took a bunch of photos in Egypt and we just really liked taking photos, and we had a website."
Their arrow-centric site has taken off: "We had over six million hits in the last 12 month period on our website," adds Charlotte.
History buffs who stumble across the site often contact the couple to ask for tips on embarking on their own arrow quests.
"They want to know if there's any arrows in their location," explains Brian.
Other photographers get in contact to let the Smiths know if they've discovered a "new" arrow. The couple will add the discovery to their encyclopedia.
The photographs range from detailed close-ups to aerial shots that depict the surrounding landscape -- mimicking what an air mail pilot would have seen back in the day.
The Smiths take aerial shots too. Pictured here: Arrow in Washington County, Utah on the Los Angeles-Salt Lake airway.
"We personally like the drone photos better because we get a better idea of what the arrow actually looks like," says Charlotte.
Photographing the arrows is often a once-in-a-lifetime experience.
"I've hiked out to them in the middle of the desert, and some just I'll never go back to them again because they're so difficult to get to," says Brian. "But I enjoy the hiking out to them and just looking at them, because we have something that nobody else does."
Brian's favorite is located in Elko County, Nevada, on the San Francisco-Salt Lake airway.
"It takes a four-wheel drive to there, as the road is dirt with loose rock and fairly steep," he recalls. "The view of the surrounding desert however was just beautiful."
This is Brian Smith's favorite arrow in Elko County, Nevada on the San Francisco-Salt Lake airway.
The Journal of Air Traffic Control reports that the beacons were officially decommissioned in the 1970s. Some four decades later, their future remains uncertain.
"I have a feeling that they're all going to become historical monuments in the next couple of years, the ones that are left," says Charlotte.
"There's been quite a couple that have been destroyed over the years and we just can look on Google Earth and see an arrow there that doesn't exist anymore, it's disheartening -- and I think the public is starting to recognize the fact that [...] it needs to be preserved," she adds.