To the south of Mare Tranquilitatis, and seperated from it by a thin spur of highlands, is the slightly smaller Mare Fecunditatis. The basic outline of the mare is circular, however to the south, the lava has overflowed the main basin, flooding some of the ajoining region.
On the left (east) edge is the prominent crater Langrenus , while the bright ring feature on the western side is the crater Goclenius. Near the centre of the mare is a bright pair of small craters with a promient ray extending westwards from one of them. These are Messier and Messier A. The bright ray stretching to the west of Messier is commonly nicknamed "the comet".