The Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, New York, was the first collider built specifically for heavy ions and the only spin-polarized proton collider ever constructed. Designed to recreate conditions shortly after the Big Bang, it enabled major studies of quark-gluon plasma and proton spin. Its double-ring machine accelerated particles to 99.995 percent of the speed of light and supported several major experiments, including STAR, PHENIX, and later sPHENIX. Among its most important findings was strong evidence that heavy-ion collisions produce an extremely hot, dense, liquid-like state of matter. RHIC ended data collection on 6 February 2026, paving the way for the future Electron-Ion Collider.