(Popular Mechanics) Our celestial neighbor holds an unending number of mysteries. What happened to Mars’ atmosphere? What did an ancient Mars look like? Could the planet have supported life? Now there’s a new question puzzling the scientists at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.,—Where did that rock come from?

The photo above, taken from the Opportunity rover, compares the same terrain on the Martian surface only 12 sols (Martian days) apart. In what is possibly the easiest spot-the-difference puzzle game ever, scientists noticed that a rock the size of a “jelly doughnut” had somehow appeared in front of the rover.

“It was a total surprise,” said lead scientist Steve Squyres in an interview with Discovery News. “We were like, ‘wait a second, that wasn’t there before, it can’t be right. Oh my god! It wasn’t there before!’ We were absolutely startled.”

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