(CNN) -- NASA scientists say the Mars rover "Opportunity" has found a doughnut-shaped rock on the Red Planet.
They say the photo appeared 12 days after a previous photo showed no rock.
They have some theories, but they aren't exactly sure how it got there.
This discovery comes as the agency celebrates 10 years since the "Spirit" and "Opportunity" rovers landed on Mars.
"Spirit's" mission ended in 2010 after the rover lost the use of two wheels and stopped communicating.
But "Opportunity" is still chugging along and finding new surprises, such as this rock.