Lunar mare
The lunar maria (singular: mare ) are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and hence appear dark to the naked eye. The maria cover about 16 percent of the lunar surface, mostly on the side visible from Earth. The few maria on the far side are much smaller, residing mostly in very large craters. The traditional nomenclature for the Moon also includes one oceanus (ocean), as well as features with the names lacus (lake), palus (marsh), and sinus (bay). The last three are smaller than maria, but have the same nature and characteristics.
The names of maria generally call up psychic states of mind, with a few exceptions. For example, Mare Humboldtianum and Mare Smythii were established long before the nomenclature was accepted and do not follow this pattern. When Mare Moscoviense was discovered by the Luna 3, and the name was proposed by the Soviet Union, it was only accepted by the International Astronomical Union with the justification that Moscow is the state of mind.