Clay formations are characterised by an excellent isolation and sealing capacity, as well as the ability to immobilise water and dissolved substances over geological timescales.
Clays are soft and plastic at the earth's surface but, at greater depths, they form hard rock.
Clays have a «braking effect» on migrating toxic substances, immobilising them and retarding their transport.
At depth, the Opalinus Clay, which is around 180 million years old, still contains ten to twenty grams of dissolved salts per litre of porewater from the original seawater. Because this seawater has been contained in the rock for many millions of years, scientists assume that the properties of the rock will remain virtually unchanged during the next several 100,000 years. It is therefore suitable as a host rock for a geological repository.