Gravity batteries are real — Energy Vault is stacking concrete blocks in Switzerland right now. But let's push it: forget block-stacking. What if we suspended one colossal weight in a deep shaft and used it to power an entire city?

Pick Pittsburgh: ~300,000 households, average draw ~1.2 kW continuous, so the city needs roughly 360 MW sustained. To run for 24 hours on stored gravitational potential energy, we need:

E = 360 MW × 86,400 s = 3.11 × 10¹³ J ≈ 8.6 GWh

Energy stored in a falling mass is E = mgh. Let's drill a shaft 2 km deep — about as deep as the deepest South African gold mines, so geotechnically plausible. Solving:

m = E / (g·h) = 3.11×10¹³ / (9.81 × 2000)
m ≈ 1.59 × 10⁹ kg = 1.59 million tonnes

If we cast it from tungsten (ρ = 19,300 kg/m³) for maximum density, the volume is ~82,000 m³ — a cylinder roughly 50 m in diameter and 42 m tall. That's a tungsten building, weighing as much as four Empire State Buildings, hanging on cables in a 2 km hole.

The cable problem. Static load on the suspension is W = mg = 1.56 × 10¹⁰ N. The strongest commercial steel cable (UHTS) tops out around 2 GPa tensile strength. Required cross-section:

A = 1.56×10¹⁰ N / 2×10⁹ Pa = 7.8 m²

That's a steel rod 3.15 m thick — and that ignores the cable's own weight over 2 km, which adds ~30% load. Even Kevlar or UHMWPE can't escape this; you're flirting with the same self-supporting-length limit that kills space elevators on Earth. Carbon nanotube fiber (theoretical 60+ GPa) would shrink the cable to 0.6 m² — which is why every gravity-storage paper eventually whispers "CNT."

The descent rate. To deliver 360 MW at, say, 90% generator efficiency:

v = P / (m·g·η) = 3.6×10⁸ / (1.59×10⁹ × 9.81 × 0.9)
v ≈ 0.0256 m/s ≈ 2.2 m/day

The weight creeps down at the speed of a sloth on sedatives. Over 24 hours it falls 2.2 m — meaning a 2 km shaft buys you nearly 2.5 years of continuous discharge before recharging. That's the upside: gravity batteries scale insanely well in duration, terribly in power density.

Recharging. Lifting 1.59 Mt back up 2 km at 80% round-trip efficiency takes ~10.8 GWh of input. A 1 GW solar farm running 11 hours a day clears it in a day. Feasible — if you can find the structural cables.

The catch nobody mentions. Heat. Even at 95% mechanical efficiency, 5% of 360 MW = 18 MW is dissipated in bearings, cable friction, and generator losses, mostly concentrated at the headframe. That's the thermal output of a small steel mill, in one building, forever. You'd need a dedicated cooling tower just for the winch room.

Energy Vault's distributed-block approach exists precisely because one giant weight creates impossible cable, bearing, and thermal problems. Physics rewards you for spreading the mass out — the universe doesn't like pinch points.