One of NASA’s most exciting ongoing missions is that of the InSight lander that currently calls Mars home. The robotic lander has the potential to teach scientists a great deal about the Red Planet and its history, but first NASA has to figure out what to do with one of its important, but underperforming, instruments.

The tool, which is described as a “self-hammering mole” is designed to dig several meters into the planet in order to take temperature readings and paint a more complete picture of what is going on deep inside. Unfortunately the instrument’s first attempt fell well short of expectations, and its engineers aren’t sure why.

As SpaceNews reports, the German team behind the self-burrowing instrument recently discussed the tool’s failure, expressing confusion over the result.

“At about 30 centimeters depth we encountered something,” Tilman Spohn of the German space agency DLR reportedly said. “We don’t know yet if it’s a harder layer of regolith or a rock.”

At this point it’s not clear if the tool rand smack dab into the middle of a rock or if the instrument itself experienced some sort of problem. The tool only managed to reach a depth of just shy of one foot, but the plan was to have it dig as deep as 16 feet. Needless to say, that’s one heck of a shortfall, and the scientists would love to know why.

Going forward, the team plans to capture images of the instrument using the lander and relay them back to Earth. The idea is to see if there’s an obvious problem with the tool, and if everything seems okay they’ll have to assume they simply ran into a rock or some other impenetrable object.

The team plans on taking another crack at digging within a few weeks, and hopefully that attempt will result in a deeper hole.