Mercury's surface is full of craters ranging from small, shallow pits to giant, complex impact basins. In addition to craters caused by impacts, volcanic and tectonic processes (i.e., endogenic processes) have created various surface features, including wrinkle ridges, lobate scarps, and smooth plains.

During MESSENGER's first Mercury flyby in January 2008, the mission's Mercury Dual Imaging System (MDIS) wide-angle camera acquired images that revealed numerous red- and orange-colored features across Mercury's surface, particularly on crater floors. These features are commonly known as "red spots". Their color, distribution, and association with specific geologic units suggest they formed relatively recently (<1 billion years ago). One idea is that these spots represent pyroclastic deposits related to volcanic eruptions. They are now officially called "hollows".

One study suggests that these unusual features are likely pyroclastic deposits associated with volcanic eruptions that occurred within the past billion years. The pyroclastic deposits may be reworked by Mercury's thin atmosphere and/or micrometeoroid bombardment, which would account for the variety of spot morphologies observed.