"The features are widespread both in latitude and longitude. Dubbed hollows, the odd landforms can be tens of meters to a few kilometers wide, whereas the impact craters that contain them are tens of kilometers wide or bigger. The hollows are often seen in clusters on the walls, floors, and peaks of the craters. Many hollows have smooth, flat bottoms and feature highly reflective material...While Mercury had previously been thought of as a geologically dead planet, with few changes to it the hollows are much smaller than known volcanic pits, and the holes appear in places on Mercury that aren't likely to have experienced volcanic activity."