Using Starships as habitats

 When I first heard about used Starships as habitats on Mars, I thought of them as still being upright, and wondered how they would shield the people living in them from radiation.  Well, the obvious answer (duh!) is to lay them on their side, and cover them with regolith (the loose gravel, rock and dust covering the surface of the Moon  and Mars).  These analysts have written a piece discussing how this can be done on the Moon, but the same principles apply on Mars.

Returning to the Moon and establishing a permanent human presence is the next step in human space exploration. This necessitates the development of lunar infrastructure up to this task. This contribution presents a framework for rapid, cost-efficient, and supporting construction of a permanent and modular lunar base within the scope of what will be technically and legally feasible today. The proposed concept uses the SpaceX Starship Human Landing System as the foundation for a lunar base. The Starship will be placed horizontally on the lunar surface and transformed into a habitable volume. A workforce of modular rovers will aid astronauts in the construction process, and an array of countermeasures are presented to protect the astronauts from the effects of exposure to radiation, lunar dust, and extended hypogravity. Psychological and psychosocial factors are included to enhance individual well-being and crew dynamics. Physical and cognitive workloads are defined and evaluated to identify effective countermeasures, including specific spacesuit requirements. The proposed construction activities are to be organized as a multinational public-private partnership to establish an international authority, a concept that has been successful on Earth but has yet to be applied to space activities on a multinational level. A roadmap incorporating each part of the construction from human and technical perspectives is outlined. Other aspects that are critical to mission success include the cultural significance of the project, legal aspects, budget, financing, and potential future uses of the base. These solutions rely mainly on existing technologies and limited modifications to the lunar lander vehicle, making it a viable solution for the construction of a lunar base in the near future.

The finished 'building' will have 2.5 times the habitable volume of the ISS.  At a fraction of the cost -- the ISS cost $150 billion, each Starship will cost $5 million according to Elon Musk, but let's say $20 million to be conservative.  The thick layer of regolith will protect against cosmic 'rays' (really, high speed particles)  and solar radiation.  And relatively quick and easy to build, too.