The Earth's upper crust is brittle, and all the tectonic deformation in this layer is accomodated by faults over a remarkable range of length scales, from a few micrometres to 1000s of kilometres. Faults are not mathematical planes (although it is often useful to idealise them as such!), and have a multiscale structure which often includes a so-called "damage zone" that can be as wide a 100s of metres, and where rocks are heavily cracked. Such fault zone structures and their properties are of paramount importance, since they control fluid flow, strength, and earthquake dynamics. Here, I will present recent laboratory rock deformation experiments that show how fault damage zones develop, and how they dynamically produce strength and fluid pressure variations. Our in situ observations reveal previously undocumented features of rock failure, such as local fluid decompression and vaporisation due to fault zone opening, which have implications for the geochemistry of fault-related mineral deposits. Such laboratory data obtained on crustal rocks under crustal pressure conditions provide unique quantitative constrains on key hydro-mechanical properties required for large-scale models of faulting.