Geological Chronicle
Mare Ingenii (Latin for 'Sea of Cleverness') is a rare far-side lunar mare spanning 318 kilometers across at 33.7° S, 163.5° E. The mare occupies the ancient Ingenii impact basin, which formed during the Pre-Nectarian epoch within the outer margins of the older and larger South Pole-Aitken basin. Because it sits entirely on the far side of the Moon, Mare Ingenii is invisible from Earth. It represents one of the few locations on the lunar far side where volcanic basalt flooded the surface in substantial volume.
During the Upper Imbrian epoch, eruptions deposited thin sheets of basalt lava across the basin floor. Unlike the deep basalt accumulations found in near-side circular maria, the volcanic layer in Mare Ingenii is shallow and incomplete, leaving underlying crustal topography partially exposed. The dark expanse is dominated by the flooded 112-kilometer-wide crater Thomson, whose rim was breached by lava that spilled eastward across the plains. Other bordering formations include Obruchev crater along the southern margin and Van de Graaff crater to the northeast.
Bright, ribbon-like lunar swirls cross the dark basaltic floor of Mare Ingenii, showing high-albedo patterns similar to Reiner Gamma on the near side. These sinuous markings correspond to strong localized crustal magnetic anomalies rather than topographic ridges or volcanic vents. Mare Ingenii is situated directly at the antipode of the Mare Imbrium impact basin. Seismic energy from the giant Imbrium impact traversed the lunar interior and focused at this antipodal region, fracturing the crust and creating furrowed terrain. The resulting crustal magnetic fields deflect solar wind ions, shielding the surface regolith from space weathering and keeping the swirls bright.
In 1959, the Soviet probe Luna 3 captured the first coarse photographs of the region, which Soviet cartographers initially designated as Mare Desiderii (Sea of Desires). After subsequent mapping revealed a smaller mare structure overlapping older highland terrain, the International Astronomical Union (IAU) formally adopted the name Mare Ingenii in 1961. High-resolution images from the Lunar Reconnaissance Orbiter (LRO) later revealed a deep lunar pit crater in the mare basalt, confirming a collapsed skylight into an underlying volcanic lava tube.