Transportation tunnel design and construction requires accepting the inherent uncertainty associated with subsurface conditions that can be characterized but never truly known. This article follows the geotechnical arc of a typical tunnel project from explorations and testing, to engineering analysis, supporting the selection of tunnel excavation methods, characterizing ground behavior, developing parameters for liner design, and estimating surface settlements and effects on adjacent structures. It provides examples from completed projects to highlight the risk and mitigation methods associated with underground construction. …