Deep geological repositories for radioactive waste rely on geological barriers to isolate waste and impede or limit radionuclide migration over long geological timescales or until radioactivity has decreased to natural radiation levels. This study summarises the main geological evidence and arguments for the containment-providing rock zone (CRZ), including the host rock Opalinus Clay, as effective geological barriers for Switzerland’s deep geological repository. The information and data are part of Nagra’s comprehensive safety case within the framework of the general licence application. The geological assessment basis contains an integrated multi-scale site characterisation, covering data from 3D-seismic investigations and nine deep boreholes from the latest exploration campaign. Highlights of the geological data include: 1) high-resolution 3D-seismic datasets that image the subsurface structure, including undisturbed structural domains, key stratigraphic horizons, and the geometry of faults; 2) a comprehensive field and laboratory dataset that demonstrates that the hydraulic conductivities of the Opalinus Clay and remaining CRZ units are extremely low, that solute transport is diffusion-dominated, and that key properties have high vertical and lateral continuity; 3) independent natural-tracer profiles that developed over millions of years and confirm slow, diffusion-dominated transport across the CRZ; 4) a laboratory-derived mechanistic understanding and field evidence of self-sealing processes in the Opalinus Clay that show that fracture transmissivities decrease relatively rapidly after mechanical perturbation; 5) a systematic abstraction approach that groups geological units with similar properties and provides a modelling framework for evaluating safety- and performance-relevant processes; and 6) broad-ranging evidence that demonstrates the long-term geological stability of the Opalinus Clay with respect to the expected tectonic, geomorphological and hydrological changes during glacial-interglacial cycles over the next one million years. The synthesis presented here demonstrates how the geological data and its abstraction provide a robust and internally consistent basis for the safety assessment supporting the Swiss general licence application.