This book provides an integrated view of underground and geotechnical engineering, combining experimental, numerical, and field studies on rock mechanics, tunnel design, mining hazards, slope stability, and subsurface storage. Studies on brittle and sedimentary rocks under thermal, dynamic, and stress conditions reveal dilation, anisotropy, and fracture processes, supported by simulations of rockburst risk, EDZs, and stress redistribution. Case studies from Indian hydroelectric and mining projects showcase site-specific designs, reinforced soil technologies, and hazard mitigation in Himalayan strata. Topics include CO2/hydrogen storage in lined caverns, hydrogeological confinement, and seepage control. Monitoring and safety are enhanced by infrared thermography, acoustic emission, and blast vibration analysis. AI-driven modeling for discontinuity mapping, stress prediction, and support optimization underscores the growing role of data-driven intelligence in resilient underground infrastructure.