Marius Hills pit (confirmed); other suspected pits at Mare Tranquillitatis, Mare Ingenii, Mare Smythii; Moon-wide candidates from skylight surveys
Hardened lava tubes from ancient volcanic flows — naturally shielded from radiation, micrometeoroids, and thermal extremes. Surface temperatures swing 127°C (sunlit) to -173°C (shadow); a few meters underground, temperatures stabilize at ~-20°C. Considered NASA's strongest candidate for long-duration crewed habitats. ESA's BR-LRD-CONOPS programme evaluates lava-tube descent architectures.
Detected but unexplored — interior dimensions, structural integrity, dust, electrostatic charging, and atmospheric composition (radon? volatiles?) are all unknown. Surface access via 50-100 m vertical skylight drop requires landing and rappelling technologies not yet demonstrated. Marius Hills is far from south pole resources and Earth-line-of-sight comms.