The radiation environment on the lunar surface is relentless. Without an atmosphere or magnetosphere, settlers face two distinct threats. Galactic cosmic rays deliver a continuous dose of approximately 380 millisieverts per year. Solar particle events can deliver several sieverts in hours during major flares. For context, the annual occupational limit for radiation workers on Earth is 50 millisieverts.

We evolved under a shield we never see. Earth's magnetosphere and atmosphere together provide shielding equivalent to roughly 10 meters of water above our heads. We have never lived without it. No human has spent more than 75 hours on the lunar surface, and those Apollo astronauts accepted radiation risks we would consider unacceptable today. To settle the Moon is to solve a problem our biology never anticipated.

Regolith is our best available shielding material because it is already there. Monte Carlo radiation transport simulations show that 2 to 3 meters of lunar regolith reduces the GCR dose to approximately 50 millisieverts per year, within occupational limits. For solar particle events, a dedicated storm shelter with 5 meters of regolith cover brings even a Carrington-class event dose below acute radiation syndrome thresholds.

Five meters of regolith. That is not a roof. That is a burial. The psychological architecture of these spaces matters as much as the physical. Humans enclosed in underground bunkers under meters of rock, with no windows and no horizon, develop specific pathologies. Depression. Claustrophobia. Time distortion. The shield that protects the body must not destroy the mind. Every habitat design must include sight lines, light simulation, and the illusion of openness.

That is why the current design integrates fiber-optic daylight piping and large-format display walls showing real-time surface camera feeds. Psychologically, a 4K display of the lunar horizon provides most of the cognitive benefits of an actual window. But the more elegant solution is lava tubes. Lunar Reconnaissance Orbiter has identified over 200 candidate skylights, some with tube diameters exceeding 500 meters. A natural lava tube provides tens of meters of basalt shielding with cathedral-scale interior volumes. If we find a structurally sound tube near the south pole, it changes everything.