The Young Ones

This high-resolution view shows hollows on the southwestern peak ring of the Scarlatti basin. The hollows are the irregularly shaped, flat-floored depressions. The image is striking because it shows that there are abundant small impact craters on the surface surrounding the hollows. But there are few if any within the hollows themselves. Since impacts occur randomly over Mercury's surface and accumulate with time, the lack of craters on the hollows indicates that they must be very young relative to the rest of Mercury's surface.

http://photojournal.jpl.nasa.gov/catalog/PIA19246