Unquestionably, the largest volume of low grade metabasites in the Earth's crust lies beneath its oceans. Extrusive rocks (and their intrusive equivalents) which are generated and metamorphosed at mid-oceanic ridges constitute the bulk of these metabasites, although in the past few years, low grade metabasites have been identified from other submarine, geodynamic settings, notably arcs and seamounts. In addition, ophiolites, which are the on-land, fossil equivalents of oceanic crust, continue to provide new data on many aspects of submarine hydrothermal metamorphism.