Human expansion beyond Earth will require the development of habitats capable of protecting settlers from radiation, vacuum exposure, extreme temperatures, micrometeoroid impacts, and the absence of breathable atmospheres. While planetary terraforming has long been proposed as a pathway toward large-scale settlement, the modification of entire planetary environments remains technologically uncertain and may require timescales extending over centuries or millennia. An alternative approach involves the creation of localized habitable environments within subsurface regions of planets, moons, asteroids, and other solid Solar System bodies.

This article examines the concept of underground terraforming, defined as the transformation of natural or engineered subsurface environments into habitable spaces through the integration of environmental control systems, life-support technologies, resource utilization, and habitat engineering. The article emphasizes current literature concerning candidate subsurface environments, including lunar and Martian lava tubes, asteroid interiors, icy moon crusts, and dwarf planets. It evaluates key engineering requirements such as excavation technologies, radiation protection, structural stability, energy systems, in-situ resource utilization (ISRU), and closed ecological life-support systems (CELSS).

Particular attention is given to the distinction between technologies that are currently demonstrated, technologies that appear plausible based on ongoing research, and concepts that remain speculative. The review also examines human factors, economic considerations, governance challenges, and major research gaps that must be addressed before large-scale underground settlement becomes feasible.

Current evidence suggests that subsurface habitats offer several advantages over planetaryscale terraforming, including reduced environmental modification requirements, natural radiation shielding, and applicability across a wide range of Solar System environments. However, substantial uncertainties remain regarding excavation scalability, long-term ecological closure, economic sustainability, and human adaptation to long-duration subsurface habitation. Consequently, underground terraforming should presently be regarded as a promising research framework rather than a demonstrated pathway toward widespread Solar System colonization.