Geological Chronicle
Lacus Somniorum, Latin for the Lake of Dreams, is the largest lunar plain designated as a lake, spanning 425 kilometers across the northeastern quadrant of the near side at 38.0° N, 29.2° E. Unlike the circular maria formed within deep impact basins, this irregular volcanic plain occupies an ancient, uneven depression. Its surface consists of Upper Imbrian basaltic lavas that flooded the region between 3.8 and 3.2 billion years ago, coalescing with the northeastern expanse of Mare Serenitatis through a broad gap northwest of the crater Posidonius.
The lava plains of Lacus Somniorum exhibit distinct spectral qualities compared to adjacent lunar seas. Spectroscopic observations indicate that the mare material contains reddish, higher-albedo soils with a lower abundance of titanium and iron than the darker basalts of Mare Serenitatis. This coloration reflects both the composition of the underlying lava flows and extensive contamination from excavated highland materials deposited across the lake floor by subsequent asteroid impacts throughout the surrounding terrain.
A series of complex geologic structures frames the borders and interior of the lake. The southern margin holds the flooded impact feature Hall and the 150-kilometer fracture Rima G. Bond. Further north, the lake wraps around the crater Daniell, which lies near the Rimae Daniell graben system. Along the rugged northern boundary, Lacus Somniorum borders the flooded craters Plana and Mason, which separate it from Lacus Mortis. Subsurface gravity measurements by NASA's GRAIL spacecraft identified a 200-kilometer buried impact crater beneath this northern border, showing that ancient impact structures guided the reach of the volcanic flooding.