Twenty years after the discovery of sea-floor hot springs, vast stretches of the global mid-ocean-ridge system remain unexplored for hydrothermal venting, The southwest Indian ridge is a particularly intriguing, region, as it is both the slowest-spreading of the main ridges(1) and the sole modern migration pathway between the diverse vent fauna of the Atlantic and Pacific oceans(2). A recent model postulates that a linear relation exists between vent frequency and spreading rate(3) and predicts vent fields to be scarcest along the slowest-spreading ridge sections, thus impeding migration and enhancing faunal diversity(2). Here, however, we report evidence of hydrothermal plumes at six locations within two 200-km-long sections of the southwest Indian ridge indicating : a higher frequency of venting than expected. These results suggest that fluxes of heat and chemicals from slow-spreading ridges may be greater than previously thought and that faunal migration along the southwest Indian ridge may serve as an important corridor for gene-flow between Pacific and Atlantic hydrothermal fields.