If you've ever driven along Interstate 10 through Casa Grande, you may have spotted a few cross-shaped structures planted in the desert landscape.

For a long time, these objects remained a mystery. In fact, throughout '60s they were part of a top secret government project, CORONA.

During the Cold War, the United States used satellite imaging to spy on the Soviet Union and China.

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However, the initial images taken by the satellites returned with poor resolution.

Thus, a grid pattern of these X's were placed in the Arizona desert for satellites to calibrate their cameras.

With the all X's being exactly 60 feet wide and 1 mile apart, these measurements were used to make the necessary adjustments in the camera before the next launch.

(Follow the evolution of image quality from CORONA)

After much trial and error to retrieve the film and develop clear images, the government had finally captured what it was after.

Many of the X's are now damaged or missing, but you can still see a few of them today.

At the center of each X, you will see an engraving that reads "$250 fine or imprisonment for disturbing this mark."

Whether this statute is still punishable may be a mystery of its own.

Corps of Engineers-US Army survey mark seen at the center of the X's. 

If you wish to venture to the Casa Grande desert, you can follow the map below to catch a glimpse of one of the structures up close.

The purple marks represents where the X's that are now missing. Yellow represents those that are damaged, and blue for the X's that are intact and still present.

(CORONA test targets map)

Maybe the easiest one to find if you are traveling from Phoenix is right off the I-10 and Jimmie Kerr Boulevard.

Exit right onto Jimmie Kerr and take an immediate left back under the I-10. Then, take a right onto South Sunland Gin Road. Once you hit Hanna Road you will see that "X marks the spot" on the southwest corner of that intersection.

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The area is somewhat desolate, but you will see multiple tire marks leading up to the X from other curious travelers.

Although the X's may not serve much purpose today, the Arizona desert can claim some credit for the national security of our country.

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