Hydrothermal seafloor massive sulphide (SMS) deposits on mid-ocean ridges (MOR) are paradoxical; their size seems to be inverse to the amount of volcanic activity. While hydrothermal SMS are more frequent at fast-spreading MOR, the largest deposits occur where volcanism appears to be a minimum. Here, at so-called 'amagmatic' segments on slow- and ultra-slow spreading ridges, ultramafic rocks from the lower-crust and upper-mantle are exhumed by long-lived faulting; a process that is thought to affect 50% of the length of slow-spreading ridges. Ultramafic-hosted seafloor massive sulphides (muSMS) in these settings form some of the largest deposits known, hosting high metal concentrations of Au, Cu, Ni, E- tech elements (Co, Pt). Whereas the magmatic driving force for volcanic-hosted SMS deposits is well established, it remains contentious for the muSMS. Similarly, while there are models for the sub-surface structure and extent of volcanic- hosted SMS, little is known about muSMS. For some muSMS, vent fluid chemistry indicates the potential for extensive sub- seafloor metal precipitation, possibly by interaction with pH barriers due to serpentinisation of the host rock. Furthermore, the physical, chemical and microbial mechanisms affecting muSMS after their formation are poorly constrained. Over the next 4 years, Project ULTRA will use deep-sea drilling, novel seismic and electromagnetic imaging techniques, borehole packers and ROV's to sample mineral deposits, rocks, fluids and microbes in an attempt to answer the questions about what processes control the formation, composition, location and fate of muSMS on slow-spreading mid-ocean ridges.