Illustration of the Mare Crisium (Sea of Crises) on the lunar surface as seen during a full Moon. Lunar maria are dark areas of basaltic plains on the Moon, formed by ancient volcanic eruptions. This lunar mare is in the Crisium basin, which formed in the Pre-Imbrian period (around 4.5 to 3.8 billion years ago). It is 555 kilometres across and is relatively flat. From 'Popular Astronomy' by Camille Flammarion (1880).