At the far eastern end is the 220-km long Rima Ariadaeus rille which continues eastward to the edge of the Mare Tranquillitatis.

[2] The northern edge of the Sinus Medii is formed by a highland region, with the impact craters Murchison and Pallas along the border.

Several flooded craters lie along this border, with Flammarion near the western edge, then Oppolzer, Réaumur, and Seeliger further east.

The English astronomer William Gilbert was the first to give a name to this mare, calling it Insula Medilunaria ("Middlemoon Island").

[3] The idea for its present name originates with Michael van Langren, who labelled it Sinus Medius in his 1645 map.