Salt domes are geology's gift to energy storage: impermeable, self-healing under stress, and already hollowed out by decades of solution mining. The U.S. Strategic Petroleum Reserve lives inside them. So let's fill one with compressed air instead of crude and see how much wind power we can bottle.

The cavern. A typical solution-mined salt cavern near Beaumont, Texas is a vertical cylinder about 70 m in diameter and 600 m tall — call it a skyscraper-shaped bubble a kilometer underground. Volume: V = π(35)² × 600 ≈ 2.3 × 10⁶ m³. That's roughly 920 Olympic pools.

How much energy fits? Salt creeps under stress, so operators cycle between a floor and ceiling pressure — typically P₁ = 45 bar and P₂ = 75 bar at that depth (overburden gradient ~0.22 bar/m). Treat the air as ideal and isothermal (the cavern walls buffer temperature). Useful stored energy:

W = P₁·V·ln(P₂/P₁)
W = 4.5×10⁶ Pa × 2.3×10⁶ m³ × ln(75/45)
W ≈ 5.3 × 10¹² J ≈ 1,470 MWh

Real CAES plants (Huntorf, McIntosh) recover about 50% round-trip because compression heats air and expansion cools it — you either waste the heat or burn gas to reheat on discharge. Adiabatic CAES, which stores compression heat in a bed of ceramic pebbles or molten salt, pushes efficiency to ~70%. Call it 1,000 MWh delivered per cavern.

Scaling to a nation. The U.S. averages ~450 GW of electricity demand. Storing one full day requires ~10,800 GWh. At 1 GWh per cavern, we'd need ~10,000 caverns. The Gulf Coast salt basin plausibly hosts a few thousand; add the Michigan basin, the Zechstein salts in Germany, and the Permian, and you're within an order of magnitude of a continental-scale battery.

Charge/discharge rate. Mass flow through the wellbore limits power. A 1 m-diameter well pushing air at Mach 0.3 (~100 m/s) at 60 bar delivers roughly:

ṁ = ρ·A·v = 70 kg/m³ × 0.785 m² × 100 m/s ≈ 5,500 kg/s

Expanding that through a turbine yields around 300 MW per well. Drill four wells per cavern: 1.2 GW discharge — comparable to a nuclear reactor, sustainable for ~1 hour.

The salt creep problem. Halite flows like extremely stiff toffee: strain rate ~10⁻¹⁰/s at cavern conditions. Over 30 years, a cavern shrinks about 1–3% by volume if pressure sags too low. Keep minimum pressure above ~30% of lithostatic and creep stays manageable. Cycling too fast, however, causes thermal spalling — the walls flake as they breathe. Practical duty cycle: one full charge/discharge per day, not per hour.

The catch. Real energy density is embarrassingly low: 5.3 TJ / 2.3 million m³ ≈ 2.3 MJ/m³, about 0.6 kWh/m³. A lithium pack stores 500× more per volume. CAES wins only because the hole is free — the salt did the excavation for us over 250 million years, and the marginal cost of another cubic meter of storage is essentially the electricity to dissolve more halite.