Soil liquefaction is a phenomenon where saturated granular soil temporarily loses strength and stiffness due to increased pore water pressure under dynamic loading, and the academic concept and research field dedicated to investigating this process. This field examines the mechanisms by which external stresses, commonly from seismic events, cause a build-up of pore water pressure that reduces effective stress to near zero, leading the soil to behave like a viscous fluid. Research within this concept focuses on understanding the factors influencing liquefaction susceptibility, predicting its occurrence and consequences, and developing methods for assessment and mitigation of the significant geotechnical hazards it poses to civil infrastructure.