A temporary workgroup of trained British soldiers fractured and nearly lost members’ lives when it encountered unexpected adversity in Low’s Gully, Borneo. Although demography-oriented theories of group faultlines and diversity types offer useful cross-sectional baselines for predicting and explaining workgroup fracture, the authors examine the Low’s Gully expedition to build theory exposing the longitudinal microfoundations of workgroup fracture under adversity. The authors incorporate the long-established concept of common ground among parties—information that is both mutually held and mutually understood to be mutually held—to uncover changes in group members’ communications success, intragroup trust, tacit coordination, and fracture likelihood as the expedition’s situation became increasingly tenuous. Their study of how this diverse workgroup faced extreme adversity shows how group trust can dissolve under adversity in a sequence moving from initial trust, to trust-with-trepidation, and then to distrust. Theoretical insights indicate that common ground arising from shared positive experiences increases workgroup resistance to fracture under adversity more than does common ground arising from similar backgrounds; a trust violation occurring among highly similar group members is perceived to be more severe and results in a greater increase in group fracture likelihood than does the same trust violation among dissimilar group members; perceived leader benevolence and integrity are more vital than ability for maintaining intragroup trust, even when ability is necessary for task success; and finally, distrust can sometimes be warranted and even vital for a workgroup’s task accomplishment. The authors discuss the implications for future research and practice.