Sinus Iridum

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.

Sinus Iridum is one of the largest craters of Upper (Late) Imbrian age.

When 10 to 11 days after new moon the Sinus Iridum is still in shadow, the Montes Jura can already be visible at the lunar terminator.

By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Promontorium Heraclides.

Sinus Iridum area (top right) in selenochromatic format holding some normal (yellow)/pyroclastic(red) selenochromatic landmarks
Detail map of Imbrium's features. Sinus Iridum is the feature marked "A".