Lacus Somniorum is a plain located in the northeastern part of the Moon's near side. It is located at selenographic coordinates 38.0° N, 29.2° E, and has a diameter of 384 kilometers. (It is the largest of the lunar features designated Lacus.) The name is Latin for Lake of Dreams, a title given to this feature by Riccioli.
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.
To the southwest this plain is joined to the Mare Serenitatis through a wide gap northwest of the crater Posidonius. This crater forms the western end of the southern border, which extends eastward to about longitude 41° before turning northwest. Along this southern border is attached the flooded crater Hall, and a 150-km-long rille named the Rima G. Bond for the small crater G. Bond south of Hall.
The irregular eastern border comes close to the small crater Maury before continuing to the north until it reaches the crater remnant Williams. From there the edge continues to the west. A narrow border region separates the Lacus Somniorum from the smaller Lacus Mortis to the north. This strip of rough terrain includes the flooded impact craters Mason and Plana.
Finally the lake curves back to the south, joining a region of rough terrain along the northern border of the Mare Serenitatis. In the southern half of this border area lies a rille system designated the Rimae Daniell. These were named for the crater Daniell, a small formation north of Posidonius that is encircled by the Lacus Somniorum. To the north of Daniell, near the northern edge of this feature, lies the small crater Grove.