I had the opportunity to visit the Tournemire underground research laboratory, which I use some of the data acquired during experimental tests of the response of a clayrock fault to fluid injection and reactivation. This site is used as an analog of a host rock or seal for geological storage. It has been of great learning to have the occasion to observe a mature fault zone in a clayrock from the inside, as questions are araised when it comes to the mechanical response, zoning, and permeability behavior of these faults that are less understood than well-known faults in more rigid lithologies (sandstones, granites).