NASA has found a very strange feature on the Martian landscape thanks to the Curiosity rover, and it was such an interesting find that the rover was sent back for a closer look. The rover found tiny tube-like structures in some rocks that may have some relation to minerals, but as of now is a mystery.

“Meanwhile, back on Mars… I’m checking out these stick-like figures,” the Curiosity Rover’s official Twitter account tweeted on Jan. 3. “Each is about a quarter-inch long. Maybe they’re crystals? Or they could be minerals that filled spaces where crystals dissolved away. Stay tuned! Science continues.”

These “tubes” are absolutely tiny, only a couple of millimeters in size, and you probably wouldn’t see them unless you were examining the rock very closely. It’s raised some questions over whether this is a geological find, or a biological one, with some suggesting that it’s possible these are trace fossils.

But that possibility is probably a long shot, considering the wide range of far more likely explanations. Unfortunately, it will be tough for us to figure out exactly what these features are as the Mars Curiosity rover isn’t equipped to really give it a thorough examination, but it adds another layer of intrigue to the Red Planet.