The central research question of this study was ‘how do actors govern the underground in large infrastructure projects?’. In this dissertation, I show that project actors grapple with the governance of the underground. Two major factors emerge in explaining the struggle. Firstly, the reductionistic philosophy of projects resulted in fragmentation of the large infrastructure project into subprojects and different phases which leads to hierarchical relationships between underground experts and project managers that are challenging to manage and leads to different priorities throughout the different project phases in which the underground can be forgotten. Secondly, the cognitive biases of project actors point toward focus on seeing underground risks predominantly as complex, which means that project participants generally believe that all risks can be known through measurements of the underground, and consequently failing to acknowledge other types of risks that require different approaches. However, accurate and reliable data on the underground was often missing, and the results show that the relationship between project managers and underground experts is important in taming the underground. Solutions to govern the underground can be sought by thinking about ways to integrate the fragmented parts of the projects, and facilitate those conditions under which knowledge integration is fostered throughout the project. The governance of the underground can be improved by project managers steering on joint ownership and inclusivity of underground experts at the different stage of the project so they are able to provide insights into the risks of the underground. Joint ownership and inclusivity might facilitate a stronger focus on the quality of the delivery of underground assets within the scope of the project realization. To accomplish ownership and inclusivity, project managers and underground experts alike need to be aware of the various types of risks that are residing in the underground.