Sinus Aestuum (latin for "Bay of Billows") forms a northeastern extension to Mare Insularum. 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 albedobasalticlava that is marked by a few small impacts and some wrinkle ridges. The eastern border is formed by an area of irregular terrain that divides the bay from the Mare Vaporum to the east. To the north is the Montes Apenninus range and the prominent crater Eratosthenes. Along the western side is the flooded crater Stadius and the Mare Insularum to the southwest.