Why is there a giant dart board on Mars?

Technically, this is called a terraced crater. It looks like there was a big impact a long time ago, and then a second one more recently almost dead center in the big one! But appearances can be deceiving, and in fact this crater is a puzzler. In the original post, I wondered if perhaps the terrain explains the weird shape; a second crater to the lower right (not visible in this cropped image, but you can see it in the bigger one) looks very similar to this main crater. But it's not at all clear what's going on here; see my original post for details.

However this weird thing formed, it's a pretty amazing image... but still edged out by the shot of sand dunes and frilly sand-slides in my Top 14 list.

Get the hi-res version here.

Image credit: NASA/JPL/University of Arizona