The Mont Terri project is an international research project with currently 22 partner organisations from 10 different countries (https://www.mont-terri.ch). The rock laboratory is located approximately 75 km to the west of the JO siting region in an anticline of the Jura Fold-and-Thrust Belt. It includes a network of underground galleries and niches in the Opalinus Clay designed for various underground experiments. More than 25 years of intensive scientific investigations have significantly increased the understanding of safety- and construction-relevant properties and processes in this rock (see Bossart et al. 2017 and references therein for an overview). The laboratory plays an important role in the development and testing of methodologies (e.g. geomechanical testing, porewater investigations; Minardi et al. 2021, Wersin et al. 2020). It also allows for in-situ testing at larger scales and over longer durations compared to deep borehole experiments (see Section 5.8.3). This report frequently refers to knowledge gained at the Mont Terri rock laboratory, taking into account the differences in the geological history and depth of the laboratory compared to the Opalinus Clay in Northern Switzerland.