Ancient observers thought that the round, dark, low areas on the face of the Moon were seas, which they called maria, surrounded by land, or terrae, and those terms are still used today. The lunar basins which contain the maria were probably formed by giant impacts very early in the Moon's history. Somewhat later, when volcanic activity began, many of them were filled with lava.

On the near side of the Moon, the visible maria cover 16 percent of the surface. However, there are other areas which have the same type of rock, but their surfaces have been blanketed by the ejecta from later impacts. These are called cryptomare. The total amount of mare (volcanic) material on the lunar near side is about 30 percent of the surface. The rocks that make up the maria most closely resemble terrestrial basalts, a type of dark, fine-grained volcanic rock. The volcanic activity that produced the maria was of a type that is not often seen on Earth flood basalts.9 They must have come from fissures that were many kilometers in length, and it is thought that the molten material had very low viscosity10- similar to that of motor oil.

Associated with many lunar maria are gravity anomalies called mascons (short for mass concentrations). These were discovered by examination of the tracking data of the lunar orbiters and confirmed by the Apollo and Clementine missions. It was found that the spacecraft would accelerate as they neared the maria, then decelerate as they moved beyond them.

The strongest mascon anomaly is associated with Mare Imbrium, although they also occur with Serenitatis, Crisium, Humorum, Nectaris, and Orientale. Clementine found additional evidence for other gravity anomalies associated with ancient, nearly obliterated basins. Although mare basalt has greater density than highland materials, this difference is insufficient to explain the size of the gravity anomaly. It is likely that the lunar crust is thinner below the maria than in other areas, and that dense mantle11 material is closer to the surface as a result. The combination of the increased density of both the surface basalt and the subsurface mantle material is the most likely cause of these features.

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