Good question! We believe at one point the sand dunes we're starting to see actually eventually completely buried the crater - which is several kilometres deep and 150km wide so quite a huge volume of sand! However, almost all of that sand has now been completely scoured away by wind over the past 3 billion years, exposing the ancient lake sediments underneath for us to study. The exact center of the crater is actually the least eroded, that's why the central peak mountain, Aeolis Mons (aeolis is latin for wind!) is so much taller than the crater floor, it's 5.5km high. It's this mountain that the rover is climbing up. In fact it's so tall its a bit of a puzzle, we're not sure why the sand formations didn't erode here.