The subsurface is one of the few domains where billion-dollar decisions are routinely made without direct observation. Geological models, reservoir simulations, production forecasts, and investment strategies all rely on incomplete data, uncertain interpretations, and evolving knowledge. Success therefore depends not simply on acquiring more information, but on making sound decisions under uncertainty. Deciding the Underground presents the Subsurface Decision Intelligence Framework (SDIF), an integrated approach that connects geology, reservoir engineering, uncertainty quantification, economics, risk, and strategic decision-making into a single decision architecture. Rather than treating these disciplines as separate technical workflows, the book demonstrates how geological uncertainty propagates through reservoir models, production forecasts, economic evaluations, and ultimately investment decisions. Going beyond conventional petroleum engineering texts, the book examines why technically sound projects can still fail, how cognitive and organizational biases shape subsurface interpretation, why history matching alone does not validate a reservoir model, and how uncertainty should be managed rather than minimized or ignored. Throughout, the emphasis is on improving decision quality—not simply building better models. Written for petroleum engineers, geologists, geophysicists, reservoir engineers, asset managers, graduate students, and industry professionals, this book bridges technical expertise with strategic thinking. It combines engineering fundamentals with practical decision frameworks applicable across the entire asset life cycle, from exploration and appraisal to field development, production, and abandonment. Key Features - Introduces the Subsurface Decision Intelligence Framework (SDIF) - Integrates geology, reservoir engineering, economics, risk, and governance into a unified decision process - Explains how uncertainty propagates throughout the field development life cycle - Examines the strengths and limitations of geological interpretation, reservoir simulation, and production forecasting - Presents practical decision frameworks alongside engineering analysis - Suitable for both graduate-level study and professional practice More than a technical reference, Deciding the Underground is a guide to making better decisions when the most valuable information remains hidden beneath the Earth's surface.