Like a blister, the “Bay of Rainbows” (Sinus Iridium) pouches out from the Ocean of Showers (Mare Imbrium) on the Moon. The bay is rimmed by the remains of the semicircular wall of a large crater seen catching the rays of the rising sun in this image. Perhaps this is the rainbow that the cartographer imagined when it was given its name. The crater was created by a huge impact on the margin of the depression of Mare Imbrium. Later, lavas flowed, layer after layer, to fill the depression and bury the southeastern rim of that crater, and created a bay-like structure on the margin of a basalt ocean.
Nearly all of this happened more than 3.8 million years ago. Since that time wrinkles developed in the solidified lava and a few smaller impacts pocked the remaining surface with small craters. A continuous rain of micrometeorites has softened the landscape somewhat, but other than that not much has happened since.
In spite of its antiquity, it remains a stark, stunningly beautiful sight, bowing, but certainly having no rain.