Extensive carbonate crusts discovered forming on the slopes of seamounts in many parts of the world's oceans are providing extra stability to the volcanic edifices. These crusts are hardgrounds composed of mixtures of volcaniclastic debris and bioclastic material, in most cases cemented by calcite, in the form of isopachous coatings around grains and pore-filling spar. Such crusts, which have been collected by a remotely-operated vehicle (ROV), are described here from moderate-depth to deeper-water slopes (180–820 m) around the volcanic island of Montserrat in the Caribbean, and from the nearby Kick'em Jenny submarine volcano off Grenada. Radiogenic 87Sr/86Sr isotope ratios from the carbonates give an indication of age (up to 0.4 Ma years old) but they also demonstrate that some samples have been altered by hydrothermal-volcanic processes, to give ages much older than expected (14 to 18 Ma) based on the foraminifera present. Such alteration is also supported by carbon and oxygen isotope (δ13C and δ18O) ratios, although most samples retain typical marine values. In many cases δ18O is usually a little more positive than expected from modern Caribbean shallow-water carbonates, likely reflecting cooler water at their moderate depths of lithification. Just one sample, from Kick'em Jenny, has very negative δ13C (−42‰) indicating methanogenesis. Crusts are also reported here from the Mediterranean Sea, with an example described from Kolumbo submarine volcano, northeast of the Santorini volcanic complex in the Hellenic subduction zone, that are similar in many respects to those from the Caribbean. Typically, the biota of the crusts consists of calcareous red algae (commonly encrusting volcanic clasts), foraminifera (benthic, some also encrusting, and planktic), subordinate serpulids, bivalves, pteropods and heteropods, and rare deeper-water corals. Some bioclasts are derived from shallower water, others from the moderate depths of the slope itself, and planktic fallout. In addition, there is evidence for the former presence of microbes from the occurrence of calcified filaments and peloids in intragranular cavities. Several generations of sponge borings are usually present as well as calcite cement. Dissolution and calcite replacement of aragonitic bioclasts and cement, and sponge spicules (originally opaline silica), have taken place. The carbonate crusts are attributed to seawater circulating within the surficial sediment, in some cases mixing with hydrothermal fluid driven by geothermal and volcanic processes. Submarine volcanic slopes clearly provide a location for moderate-depth carbonate production and cementation, but a further significance of these hardgrounds is in stabilising seamounts, enabling their slopes to avoid frequent collapse, dissection and readjustment. However, when failure does occur, larger-scale submarine landslides involving coherent slabs are more likely.