When observing the Moon, you can see dark spots on its surface. In the old days, these were thought to be the seas and oceans just like on Earth, so they were called »mare« (plural: »maria«). Giovanni Riccioli (1598–1671), an Italian priest and astronomer was leading in this area and was responsible for giving many lunar maria their Latin or partially poetic names. We would like to single out a few that we consider to be worthwhile mentioning.
The Mare Crisium , the sea of dangers, is a clearly visible, round spot, which lies isolated on the edge and is perfectly suitable to locate the position of the maria on the surface of the Moon, as this keeps on changing during the course of a night and also in relation to the geographic location.
Immediately next to it – likewise, in the shape of three round spots – the Mare Serenitatis , the Mare Tranquillitatis  and the Mare Fecunditatis , thus serenity, tranquillity and fertility. A beautiful triad that instantly reflects in human life.
Finally, we would still like to mention the Oceanus Procellarum , the ocean storms, a large area opposite the Mare Crisium and the only »ocean« on the Moon.
In actual fact, the maria are lava, meaning molten rock (magma), which surfaced from the inside of the Moon and solidified. This happened through impacts of meteorites during the origin of the Moon, which also explains why the front is mainly affected (16.9% of the surface) and less the reverse side of the Moon (only 2.6% of the surface).
The lunar maria are often interpreted as figures, like the face or the hare on the Moon. Humans simply have a flourishing imagination.