Any flat, low, dark plain on the Moon. Maria are huge impact basins containing lava flows marked by ridges, depressions (graben), and faults; though mare means sea in Latin, they lack water. The best-known is probably Mare Tranquillitatis (Sea of Tranquillity), the site of the Apollo 11 manned Moon landing. Most of the approximately 20 major maria are on the side of the Moon that always faces Earth; they are its largest surface features and can be seen from Earth with the unaided eye. The dark features of the man in the moon are maria.