Today's Image of Mars shows what was once an island in the middle of a Maja Valles, a large sea on Mars, as taken by THEMIS aboard the Mars Odyssey Orbiter. The valley was created by a huge torrent of water likely released from a ruptured underground aquifer. If you can imagine, the water came rushing into the area, pushing everything in it's path aside and leaving the relatively smooth surface we see below.
The small island we see below was terrain raised high enough above the water that it wasn't covered. We can see that at some point this island may have been hit by an asteroid near it's bottom tip. If the asteroid hit while there was still water present that would explain why there are no visible remnants of the impact anywhere else but on the island.
Clicking on this image will take you to the original high resolution image from THEMIS.