<p>Earth is the only known planet where plate tectonics and surface water coexist. Although water represents only a small fraction of the entire Earth’s mass, it exerts a profound influence on geodynamics and global cycling of elements. This coupling is most effective in subduction zones, where burial of seawater-altered oceanic lithosphere into the mantle is accompanied by mineral dehydration reactions and release of aqueous fluids. These fluids trigger mantle metasomatism, arc magmatism and generation of continental crust, while contributing to the long-term return of volatiles to the surface and sustaining planetary habitability. Deep fluids also affect the tempo of geological processes. Recent studies indicate that fluid release and fluid–rock interaction at depth are not necessarily slow, steady-state processes lasting millions of years. Instead, they may occur through as spatially localized, transient pulses which can unfold in less than one million years, while reaction halos around metamorphic veins indicate that fluid–rock interaction may occur over timescales of weeks to months. These durations approach the timescales of seismic cycles and deformation transients and are orders of magnitude faster than plate motions and long-term metamorphic evolutions. Transformative processes in the deep Earth can therefore couple with short-lived mechanical events and operate on timescales comparable to those of human lives. This emerging view challenges the traditional perception of geological processes as uniformly slow; rather, the deep Earth appears as a dynamic system in which fluid-mediated transformations occur through episodic, rapid events. Recognizing the pulsed nature of deep fluid flow has important implications for models of element recycling, mantle heterogeneity, and the temporal framework of geodynamic processes, as well as for understanding the feedback between the solid Earth and the conditions that sustain life.</p> Graphical abstract <p></p>