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(Freely adapted from Wikipedia)
Lacus Somniorum is a plain located in the northeastern part of the Moon's near side and has a diameter of 384 kilometers. (It is the largest of the lunar features designated Lacus.) Lacus Somniorum is an irregular feature with complex, somewhat ill-defined borders. The surface has the same low albedo as the larger lunar mare found on the Moon, and its surface was formed by flows of basaltic lava. A narrow border region separates the Lacus Somniorum from the smaller Lacus Mortis to the north.
Main formations:
Lacus Mortis, latin for "Lake of Death", (diam. 151 Km) is a plain of basaltic lava flows in the northeastern part of the Moon. It lies just to the south of the elongated Mare Frigoris, being separated by a slender arm of rugged ground. Located prominently just to the east of the midpoint of this feature is the crater Bürg. The western part of the Lacus Mortis contains an extensive system of criss-crossing rilles collectively designated Rimae Bürg.
Posidonius (diam. 95 Km, depth 2.3 Km) is a lunar impact crater that is located on the north-eastern edge of Mare Serenitatis, to the south of Lacus Somniorum. The rim of Posidonius is shallow and obscured, especially on the western edge, and the interior has been overlain by a lava flow in the past. The crater ramparts can still be observed to the south and east of the crater rim, and to a lesser degree to the north. There is a smaller, semi-circular rim of a concentric, flooded crater within the main rim, offset towards the eastern edge. There is no central peak, but the floor is hilly and laced with a rille system named the Rimae Posidonius.
Atlas is a prominent lunar impact crater (diam. 87 Km, depth 2.0 Km) that is located in the northeast part of the Moon, to the southeast of Mare Frigoris. The inner wall of Atlas is multiply terraced and the edge slumped, forming a sharp-edged lip. This is a floor-fractured crater with a rough and hilly interior that has a lighter albedo than the surroundings. Floor-fractures are usually created as a result of volcanic modifications. There are two dark patches along the inner edge of the walls; one along the north edge and another besides the southeast edges. A system of slender clefts named the Rimae Atlas crosses the crater floor, and were created by volcanism.
Hercules is a prominent crater located in the northeast part of the Moon, (diam. 69 Km, depth 3.2 Km) to the west of the crater Atlas. It lies along the east edge of a southward extension in the Mare Frigoris. To the west across the mare is Bürg. The interior walls of Hercules have multiple terraces, and there is a small outer rampart. The crater floor has been flooded by lava in the past, and contains several areas of low albedo. In the past this crater has been reported as the site of some transient lunar phenomenon.
Bürg is a prominent lunar impact crater (diam. 87 Km, depth 2.0 Km) in the northeast part of the Moon. It lies within the lava-flooded, ruined crater formation designated Lacus Mortis. The rim of Bürg is nearly circular with relatively little wear. The interior is bowl-shaped, and there is a large central mountain at the midpoint.
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