This book presents state-of-the-art research in rock mechanics, tunnel/geotechnical engineering, addressing infrastructure, energy, and hazard-prone environments. Contributions range from laboratory studies on granite, sandstone, shale, and gypsum to advanced simulations capturing thermo-hydro-mechanical responses, fracture propagation, and fluid migration. Special emphasis is placed on coupled processes, including hydraulic fracturing, discrete fracture networks, grain-based simulations, and phase-field methods. Digital innovations such as VR tunnel visualization, LiDAR, 3D point clouds, and AI-driven prediction enable real-time monitoring and smart decision-making. Case studies from the Himalayas, Nepal, Norway, and Taiwan highlight seismic risks, slope instabilities, and stress redistribution. By integrating experiments, field data, and high-fidelity modeling, this book equips engineers and researchers with innovative approaches for resilient, safe, and sustainable underground development.