This white paper introduces LUGES — Lunar Underground Gravitational Energy Storage — a proposed low-gravity megastructure energy-storage and civil-engineering architecture for mature lunar settlements. The concept stores energy by lifting locally manufactured mass modules through deep vertical shafts and recovers energy through controlled descent. It explores dual-gallery underground operation, lunar day/night energy strategy, ISRU-derived mass modules, hybrid integration with solar, nuclear, batteries, flywheels, regenerative fuel cells, and settlement-scale civil infrastructure.