Sinus Iridum (Latin for "Bay of Rainbows") is a plain of basalticlava that forms a northwestern extension to the Mare Imbrium on Earth's moon. It is surrounded from the northeast to the southwest by the Montes Jura range. The protruding part of the range at the southwest end is named Promontorium Heraclides, while that at the northeast end is called Promontorium Laplace. This bay and the surrounding mountains is considered one of the most beautiful features on the Moon, and is a favorite among lunar observers.
Sinus Iridum is formed from the remains of a large impact crater, which was subsequently flooded with basaltic lava, inundating the "sea" wall. The bay itself does not contain any notable impact craters, but does include the satellite crater Heraclides E in the south, Laplace A along the eastern edge, and Bianchini G in the north. The surface is level, but is marked by a number of wrinkle ridges (dorsa).