The obvious question about a concrete house that sinks: why not build it out of something stronger? The answer is that "stronger" is the wrong metric. An externally pressurised sphere fails by buckling, and buckling resistance is set by stiffness and geometry, not raw strength. Once you normalise for that, the material that gives you a safe hull for the least money is the humble one. Play with the metric below and see for yourself.
Eight hull candidates, from fibre-reinforced concrete to titanium. Pick a metric and the chart re-ranks, best first. The headline metric is relative hull cost at equal buckling resistance: the true cost of a shell that resists implosion equally well, not the price of a kilogram.
A thin sphere under external pressure buckles when P_cr ≈ 1.21·E·(t/R)². To give two materials the same buckling margin at the same radius, wall thickness must scale as t ∝ 1/√E - a stiffer material can be thinner. The hull's mass then goes as ρ/√E, and its material cost as ρ · $/kg / √E. That single index, normalised to concrete, is the honest way to compare.
Concrete is heavy and not very stiff, so it needs a thick wall - but at ~$0.15/kg it is one to two orders of magnitude cheaper per kg than the alternatives. The cost of the extra thickness is swamped by the cost saving per kilogram. Steel lands ~10× the concrete hull cost, aluminium ~17×, carbon ~80×, titanium hundreds of times over. For a shallow habitat where wall thickness is driven by buildability anyway, concrete is not a compromise - it is the optimum.
Compressive strength against raw cost per kilogram, log scale; bubble size is density. The bottom-left is cheap-and-weak (concrete), the top-right is strong-and-expensive (titanium, carbon). For a shallow sphere you want the cheapest thing that clears the low pressure bar, which pulls you hard to the left.
Representative 2026 handbook figures, ranges collapsed to a single value. Compression is the governing strength for an externally pressurised hull. The hull-cost index is the equal-buckling cost normalised to concrete = 1.
| Material | ρ (kg/m³) | Comp (MPa) | Tens (MPa) | E (GPa) | $/kg | Hull cost × | Corrosion |
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