Planetary scientists have long noted the possibility of large lava tunnel systems on the Moon and Mars, similar to the lava tunnels formed by terrestrial volcanism but far more ancient in origin and vastly larger –some kilometers wide and extending for thousands in length. It has often been suggested that these could be used as the basis of large habitats, being initially used as simple cave shelters for pneumatic habitat modules then later sealed up and pressurized whole as vast habitat chambers. Since this would be one of the easiest and most economical approaches to building a large colony habitat, it is very likely that this approach would be pursued and, though chances are low that these would be found exactly the right locations, adapting to a slightly less than ideal location would be well compensated by the savings in time, effort, and resources for their use.
Conversion of lava tunnels into habitats would be essentially the same as the construction of Atrium Habitats with the benefit of having most of the space already hollowed out and with the addition of employing ‘regolete’ (regolith concrete) to seal any large openings. Extensive geophysical analysis and robotic exploration of the lava tunnels would be necessary to confirm their structural integrity before use. Once confirmed safe, some minor excavation work may be employed to refine the lava tunnel wall surfaces for use and install the same kind of socket grid employed in other habitats for the mounting of habitat structures and skysim paneling. So large would these lava tunnels be in volume that skysim panels may need to be limited to bright lamp portals rather than covering the entire ceiling surface, vast atmosphere plants would be needed to initially pressurize them, and large lengths of tunnel would be dedicated entirely to the role of CELSS.
Dwellings would be largely the same as those of the atrium habitats and Aquarian housing, the sloped sides of the tunnels terraced and carved into arrays of habitat chambers of more uniform topology and retrofit with the Utilihab-style panel systems. So large are these tunnels that they would most certainly be used as communal habitats, potentially hosting hundreds of thousands of inhabitants and large diverse biomes. They are even likely to be host to light aircraft and the winged human flyers so often depicted in lunar settlement visions.
Additional atrium habitats as well as grid vaulted industrial complexes are likely to be constructed on the tunnel flanks, these vast channels becoming the primary avenue linking very large colony complexes. As with atrium habitats, vast solar, heliostat, and radiator arrays would be deployed along the surface above them.
Lava tunnels are likely to be much ‘rougher’ in landscape when terraformed, the rock floor left in a natural state when cultivated and the overall forms creating the impression of long canyon valleys large enough that their own weather systems might develop. These may prove to be the key proving grounds for early terraforming research.
Ultimately, lava tunnels could be the largest individual habitat structures deployed in surface colonization in the solar system, though it remains a very speculative concept as very little is actually known about the disposition and structure of these tunnels. When/if found near telerobotic settlements, they are likely to become the immediate focus of research attention because of the potential as habitats. Finding one of these in a providential location well suited to access to orbit and resources would be a major boon to a colonization effort, perhaps shaving decades off the time it takes to create permanent human settlements.