Bad Astronomy is reporting Mars is depressed. NASA has found what appears to be a sinkhole on Mars. It looks pretty sweet, especially if you have access to 3-D glasses.
It probably is not a crater because it is lacking certain features (no raised rim, for example). HiRISE is interpreting this to be a result of magma-water interactions. I'm not an expert (so take what I say with the appropriate skeptical position), but I will reserve judgment on what caused this feature until there is more data than a picture.
I can't find any information on the exact scale of the object, but those are DUNES in the center of the depression! I also am a fan of the pull-apart features lining the rim of the depression. It reminds me of a project in my undergrad where we mapped a landslide along the local reservoir. Well, enjoy tracking down a pair of red-green glasses, and staring at your monitor like a little kid. Science is rad.