Literally sea, a term used for extensive dark areas on the Moon. Its use dates from a time when it was believed that the darker features on the Moon were of liquid water; this is now known to be untrue. In recognition of its use over a long period, the term has been retained in official names for these features on the Moon. The largest of the maria is called ocean oceanus rather than sea - Oceanus Procellarum. The lunar maria are actually seas of solidified lava, dating from the era shortly after its formation more than 4,000 million years ago when the Moon was volcanically active. The molten lava flowed into huge basins that had been excavated by the impacts of large meteorites. By this stage in the Moon’s history, the frequency of meteoritic impacts had fallen. There is a noticeably lower density of craters on the lunar maria than on the brighter highland areas terrae.