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