Opalinus Clay is an argillaceous rock which was deposited 174 million years ago in a shallow sea during the Jurassic period. In the Opalinus Clay fossil shells of the ammonite genus Leioceras opalinum are widespread. The name derives from the shimmering (opalescent) lustre of the shells.

A rock laboratory offers more realistic test conditions than is the case with conventional laboratory studies. Experiments can be carried out here on a 1:1 scale. The experiments in the rock laboratory provide essential knowledge about the feasibility and safety of a deep geological repository. Rock laboratories alone, however, are not enough. To evaluate the long-term safety of a deep storage facility (i.e. a safety analysis) it is also necessary to study natural analogues, models, and on-the-spot investigations.

An idea for a test is checked using projections and calculations to assess its feasibility. If it is clear that the objectives cannot be achieved with conventional laboratory tests, a test location in the rock laboratory is agreed upon. Before the test can be carried out, the start-up conditions and constraints are ascertained at the test location itself. Only then can activities such as drilling, setting up instrumentation, and measuring be initiated. Months or even years later, the results are evaluated and compared with the projections.

Opalinus clay exhibits several properties that have a beneficial effect on the safety of geological deep storage. These include: good potential for embedding, very low water permeability, mainly diffusion transfer of dissolved substances, a homogeneous structure, retention of radionuclides at clay mineral surfaces, and the ability to swell up and thereby seal cracks and fissures.