This image shows a very dark spot on an otherwise bright dusty lava plain to the northeast of Arsia Mons, one of the four giant Tharsis volcanoes.
This is not an impact crater as it lacks a raised rim or ejecta. What's amazing is that we cannot see any detail in the shadow! The cutout shows this dark spot and a version that is "stretched" to best see the darkest area, yet we still cannot see details except noise.
The HiRISE camera is very sensitive and we can see details in almost any shadow on Mars, but not here. We also cannot see the deep walls of the pit. The best interpretation is that this is a collapse pit into a cavern or at least a pit with overhanging walls. We cannot see the walls because they are either perfectly vertical and extremely dark or, more likely, overhanging.
The pit must be very deep to prevent detection of the floor from skylight, which is quite bright on Mars.
Image PSP_003647_1745 was taken by the High Resolution Imaging Science Experiment (HiRISE) camera onboard the Mars Reconnaissance Orbiter spacecraft on 07-May-2007. The complete image is centered at -5.5 degrees latitude, 241.4 degrees East longitude. The range to the target site was 252.5 km (157.8 miles). At this distance the image scale is 25.3 cm/pixel (with 1 x 1 binning) so objects ~76 cm across are resolved.
The image shown here has been map-projected to 25 cm/pixel and north is up. The image was taken at a local Mars time of 03:27 PM and the scene is illuminated from the west with a solar incidence angle of 52 degrees, thus the sun was about 38 degrees above the horizon. At a solar longitude of 233.4 degrees, the season on Mars is Northern Autumn.