This is an MDIS mosaic of the mighty Caloris basin, Mercury's youngest large impact basin. Caloris has been filled by volcanic plains that are distinctive in color from the surrounding terrain. Subsequent craters have excavated low-reflectance material from beneath these volcanic plains, possibly giving clues to the composition of the basin floor. The basin interior has a complex tectonic history. The interior smooth plains have an area approximately the area of Alaska! Over 640 Rhode Islands would fit inside of this massive basin. http://photojournal.jpl.nasa.gov/catalog/PIA19213