In this HiRISE captured an image of a black spot mars the flank of Arsia Mons that is most likely a portal innto a subterranean cavern. Their identifications were based upon Mars Odyssey THEMIS images, which achieve resolutions of a little better than 20 meters per pixel; having spotted the caves, they requested that the sharper-eyed HiRISE camera on Mars Reconnaissance Orbiter target the spots for more detailed imaging. The image above is the first one of these, and it shows the cave entrance called Jeanne.
There is absolutely nothing visible inside that hole. HiRISE is a very sensitive instrument, but it seems that the cave is so big and so deep that almost none of the light that enters the cave comes out. It's deep, and it's big; the hole that we see really is just a skylight on a big subterranean room. We'll never know for sure how big or deep the hole is without a manned mission visiting it, but the HiRISE team will calculate the sensitivity of the camera to put a lower limit on how deep that cave must be for HiRISE to be able to see nothing at all inside it.
Image credit: Credit: NASA / JPL / U. Arizona.