By the time Curiosity’s current location formed, the long-lived lakes were gone in Gale Crater, the rover’s landing area, but water was still percolating under the surface­. The rover found dramatic evidence of that groundwater when it encountered crisscrossing low ridges, some just a few inches tall, arranged in what geologists call a boxwork pattern. The bedrock below these ridges likely formed when groundwater trickling through the rock left behind minerals that accumulated in those cracks and fissures, hardening and becoming cementlike. Eons of sandblasting by Martian wind wore away the rock but not the minerals, revealing networks of resistant ridges within.

The ridges Curiosity has seen so far look a bit like a crumbling curb. The boxwork patterns stretch across miles of a layer on Mount Sharp, a 3-mile-tall (5-kilometer-tall) mountain whose foothills the rover has been climbing since 2014. Intriguingly, boxwork patterns haven’t been spotted anywhere else on the mountain, either by Curiosity or orbiters passing overhead.