Sinus Aestuum forms a northeastern extension to Mare Insularum

Mare Insularum

Mare Insularum is a lunar mare located in the Insularum basin just south of Mare Imbrium. The basin material is of the Lower Imbrian epoch, with the mare material of the Upper Imbrian epoch. The mare is bordered by the craters Copernicus on the east, and Kepler on the west...

. It has selenographic coordinates 10.9° N, 8.8° W, and it lies within a diameter of 290 km.

The Sinus Aestuum is a level, nearly featureless surface of low albedo

Albedo

Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

 basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

ic lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

 that is marked by a few small impacts and some wrinkle ridges. The eastern border is formed by an area of irregular terrain

Terrain

Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...

 that divides the bay from the Mare Vaporum

Mare Vaporum

Mare Vaporum is a lunar mare located between the southwest rim of Mare Serenitatis and the southeast rim of Mare Imbrium. The lunar material surrounding the mare is from the Lower Imbrian epoch, and the mare material is from the Eratosthenian epoch. The mare lies in an old basin or crater that is...

 to the east. To the north is the Montes Apenninus

Montes Apenninus

Montes Apenninus are a rugged mountain range on the northern part of the Moon's near side. They are named after the Apennine Mountains in Italy....

 range and the prominent crater Eratosthenes

Eratosthenes (crater)

Eratosthenes is a relatively deep lunar impact crater that lies on the boundary between the Mare Imbrium and Sinus Aestuum mare regions. It forms the western terminus of the Montes Apenninus mountain range. The crater has a well-defined circular rim, terraced inner wall, central mountain peaks, an...

. Along the western side is the flooded crater Stadius

Stadius (crater)

Stadius is a ghostly remnant of an ancient lunar crater that has been nearly obliterated by basaltic lava flows. It lies to the southwest of the much younger crater Eratosthenes, at the north edge of Mare Insularum where the mare joins Sinus Aestuum. To the west is the prominent ray crater...

and the Mare Insularum to the southwest.