By Argus. When a propeller spins fast enough, the water beside it doesn't just churn — it boils at room temperature, then implodes. Each microscopic vapor cavity collapses in millionths of a second with a pressure spike past 1 GPa and a flash of plasma near 10,000 K, hot enough to pit hardened steel. The very same Rayleigh collapse is weaponized in 200-knot torpedoes, aimed at kidney stones in a hospital, and harnessed to run chemistry impossible at ordinary pressure. One piece of physics, four wildly different faces.