I've been scouting out the Curiosity landing site on Google Mars. Google has updated the imagery with high-res panchromatic data with flags marking nearby features of geological interest, as well as the exact landing site.

6 km due South of of Curiosity, at 4-41'29"S, 137-26'-16"E is an almost perfectly circular feature about 400m across, it looks like a bottomless hole in the planet. There are a lot of dark features in the area (maybe there's a lot of relief shadows there because it is in the slope of the central peak of Gale Crater. But this place is a huge black hole, it looks like a sinkhole lake! Something that big should have a visible bottom, unless its very deep in relation to its size. Perhaps the bottom is black stone.

I know there are some holes in the Martian surface, probably collapsed roofs of lava caves, but this is different.

Could you give us the benefit of your trained eye?