Geological Chronicle
Lacus Mortis, Latin for the Lake of Death, is a roughly hexagonal plain of volcanic basalt in the northeastern quadrant of the Moon's near side at 45.1° N, 27.3° E. Spanning 151 kilometers in diameter, the structure originated as a floor-fractured impact basin during the Pre-Imbrian epoch and filled with basaltic lavas during the Late Imbrian period. The lake lies south of Mare Frigoris, separated by an arm of rugged highland terrain, and north of Lacus Somniorum, bounded by the ancient degraded craters Plana and Mason.
The most prominent landmark within the plain is Bürg, a 40-kilometer complex crater formed during the Copernican period approximately 800 million years ago. Bürg has steep, terraced walls and a sharp central peak rising from its bowl. Extending west from Bürg is Rimae Bürg, a network of graben and rilles spanning 140 kilometers across the lava floor. Planetary geologists interpret these rilles as collapsed lava tubes and tectonic fractures formed during volcanic cooling.
The lake floor holds several volcanic and structural formations. West of Bürg at 44.96° N, 25.62° E lies a pit crater measuring 100 by 150 meters and descending 90 meters deep, where a collapsed eastern wall forms an inclined ramp down to the floor. Near the southwestern border, two small volcanic cones measure 1.5 kilometers across, resembling terrestrial cinder cones. The surrounding basalts contain higher concentrations of magnesium oxide and aluminum oxide than typical lunar maria, alongside low titanium concentrations under 1.5 percent.
Italian astronomer Giovanni Battista Riccioli named the formation Lacus Mortis in his 1651 map *Almagestum Novum*, a term formally adopted by the International Astronomical Union in 1935. In modern lunar exploration, Astrobotic selected Lacus Mortis as the target landing region for its Peregrine Mission 1 lander, directing attention to the plain's volcanic pits and rille systems as candidates for subsurface cave exploration.