Geological disposal has been proposed as the solution for managing the most radioactive nuclear wastes. These wastes stay hazardous for periods better measured in geological rather than human time. This challenges the capacity of science to accurately predict the future evolution and safety of geological disposal facilities. Thus nuclear waste management organisations, such as the Finnish Posiva, utilise a broad range of historical and cultural evidence and discursive tactics to support their scientific argumentation for the long-term safety of geological disposal. Posiva, for instance, makes its safety case by anchoring its disposal project in broader international scientific settings and engineering practices; by giving the abstract disposal concept form and tangible through mundane objects and spaces and in so doing presenting geological disposal as an extension of the everyday; and finally by juxtaposing the underground and the aboveground to depict the underground as more predictable and knowable, essentially as ‘safer than.’ By invoking and imagining geological disposal through the familiar, Posiva negotiates the limits of scientific knowledge, and undermines the nuclearity of geological disposal to make its case for the implementability, manageability and safety of geological disposal and, crucially, to present geological disposal as the only available option for long-term nuclear waste management.