Lacus Mortis, latin for "Lake of Death", 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. To the south is the Lacus Somniorum, separated from this mare by the joined craters Plana and Mason, and a strip of uneven surface.
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.
The selenographic coordinates of the Lacus Mortis are 45.0° N, 27.2° E, and it has a diameter of 151 km.