The dark mares on the moon were formed by ancient volcanic activity.

mares 是拉丁语借词，原意为"海洋"。在月球地质学中，这个词被用来指代月球表面那些较暗的平坦区域，这些区域在古代被误认为是海洋，实际上是由古代火山喷发形成的玄武岩平原。