Mare Nubium is a lunar mare

Lunar mare

The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...

 in the Nubium basin on the Moon's near side. The mare is located just to the southeast of Oceanus Procellarum

Oceanus Procellarum

Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...

. The actual basin is believed to be of Pre-Nectarian

Pre-Nectarian

The Pre-Nectarian Period of the lunar geologic timescale runs from 4533 million years ago to 3920 million years ago, when the Nectaris Basin was formed by a large impact. It is followed by the Nectarian period...

 system, with the surrounding basin material being of the Lower Imbrian

Lower Imbrian

In the lunar geologic timescale, the Early Imbrian epoch occurred between 3850 million years ago to about 3800 million years ago. It overlaps the end of the Late Heavy Bombardment of the inner solar system. The impact which created the huge Mare Imbrium basin occurred at the start of the epoch...

 epoch. The mare material is of the Upper Imbrian

Upper Imbrian

In the Lunar geologic timescale, the Late Imbrian epoch occurred between 3800 million years ago to about 3200 million years ago. It was the epoch during which the mantle below the lunar basins partially melted and filled them with basalt...

 epoch. The crater to the west of the mare is Bullialdus

Bullialdus (crater)

Bullialdus is a lunar impact crater located in the western part of the Mare Nubium. To the north by north-west is the broken-rimmed and lava-flooded crater Lubiniezky. South-west of Bullialdus lies the smaller crater König....

, which is of the Eratosthenian

Eratosthenian

The Eratosthenian period in the lunar geologic timescale runs from 3,200 million years ago to 1,100 million years ago. It is named after the crater Eratosthenes, whose formation marks the beginning of this period. The formation of the crater Copernicus marks its end, and the beginning of the...

 Epoch. This means the crater is younger than the mare it sits in. The crater that sits on the southern rim of the mare is Pitatus

Pitatus (crater)

Pitatus is an ancient lunar impact crater located at the southern edge of Mare Nubium. Joined to the northwest rim is the crater Hesiodus, and the two are joined by a narrow cleft. To the south lie the attached Wurzelbauer and Gauricus....

. The first released images coming from the Lunar Reconnaissance Orbiter

Lunar Reconnaissance Orbiter

The Lunar Precursor Robotic Program is a program of robotic spacecraft missions which NASA will use to prepare for future human spaceflight missions to the Moon. Two LPRP missions, the Lunar Reconnaissance Orbiter and the Lunar Crater Observation and Sensing Satellite , were launched in June 2009...

were of the Mare Nubium.