Superconducting magnetic energy storage (SMES) already exists — refrigerator-sized units park a few MJ inside cryogenically cooled coils to buffer grid transients with millisecond response. The energy lives in the magnetic field itself: U = ½LI². Bigger loop, bigger inductance, bigger stored energy. So what happens if we make the loop the whole planet?

Lay a single-turn HTS cable around the equator — 40,075 km of REBCO-coated tape immersed in a liquid-nitrogen jacket, following the ITER-scale magnet playbook. Take the loop radius as Earth's radius, r = 6.371 × 10⁶ m, with a cable bundle radius a = 1 m. Standard thin-wire formula:

L ≈ μ₀·r·[ln(8r/a) − 2]
  ≈ (4π×10⁻⁷)(6.371×10⁶)(ln(5.1×10⁷) − 2)
  ≈ 126 henries

126 H is a lot of inductance. Now the current. If we push commercial-HTS-tape density (~10⁹ A/m²) through a 1 m² cross-section: I ≈ 3.1 × 10⁹ amps. Plug in:

U = ½ × 126 × (3.14×10⁹)²  ≈  6.2 × 10²⁰ J  ≈  172,000 TWh

That's roughly six years of global electricity consumption stored in a single hoop of field lines. Marvelous — except the ring instantly detonates.

The hoop-stress problem. At the cable surface, B = μ₀I/(2πa) ≈ 628 T. Magnetic pressure scales as B²/2μ₀, giving 157 GPa pushing outward on every square meter of conductor jacket. Maraging steel yields around 2 GPa. The ring would peel itself off the equator faster than any structural material can resist — and no known superconductor even keeps its critical current above ~45 T anyway.

Dial back to a physically survivable B = 50 T at the cable (the frontier of what HTS bulk magnets have hit in labs). That drops the current to I ≈ 2.5 × 10⁸ A, so:

U = ½ × 126 × (2.5×10⁸)²  ≈  3.9 × 10¹⁸ J  ≈  1,100 TWh

Still absurd — about 13 days of world electricity demand stored magnetically, all discharged through the same two terminals. Magnetic pressure is now ~1 GPa, borderline achievable if you jacket the cable in wound Kevlar-and-steel bandages the way pulsed-field magnets already do.

The bill. HTS tape runs ~$50/(kA·m). For 250,000 kA over 40,000,000 m: ~$500 trillion just for conductor — five times world GDP. Cryogenics alone dwarf that: keeping a 40,000 km linear cryostat at 77 K, given even 0.5 W/m parasitic heat leak, means 20 MW continuous refrigeration — trivial compared to the stored energy, but a permanent industrial load spanning ocean floors, mountains, and geopolitically hostile terrain.

And the failure mode. A local quench — one spot going normal-conducting — dumps 3.9 EJ into a resistive hot zone. That's ~900 megatons TNT-equivalent, or 60 Tsar Bombas, released as a joule-heating event and a collapsing magnetic field whipping induced currents through every conductor on the planet. You'd need thousands of persistent-current switches to compartmentalize the loop into safely-dumpable sections — and any single dumped section would still vent tens of PJ locally.

The ring "works" as physics; it fails as engineering because the energy density of a strong magnetic field is a stored-explosive density.