Observation of the strange features discovered by the HiRISE camera on NASA's Mars Reconaissance Orbiter (MRO) at the southern edge of Acidalia Planitia on Mars. The main cluster of pits on the left side of the photo are approximately 500 meters long and 100 meters wide.

We may be routinely orbiting, roving, drilling and lasing Mars, searching for elusive traces of life and reconnoitering sites for future human missions, but that doesn't mean studies of the red planet don't throw up surprises. On the contrary.

Take this March 21, 2013 observation by the High-Resolution Imaging Science Experiment (HiRISE) camera on board NASA's Mars Reconnaissance Orbiter (MRO) of the southern edge of Acidalia Planitia, a plain located in the planet's northern hemisphere.

These irregular depressions with weird raised rims aren't impact craters and they can't be wind-blown features as the pits contain boulders that could not have been moved by the Martian winds. HiRISE mission scientists don't believe they could be caused by volcanism either.