The possibility of mining seafloor massive sulfide deposits has stirred debate about the sustainable use of this new resource and whether commercial development is worth the risk. Among the outstanding questions is how many deposits might be accessible to deep-sea mining. More than 300 sites of high-temperature hydrothermal venting have been identified since the discovery of black smokers, but significant massive sulfide accumulation has been found at only 165 of these sites. Estimates of the total number of vent fields and associated mineral deposits, based on plume studies and deposit occurrence models, range from 500 to 5000. We have used new deposit occurrence data from 10,000 km of ridge, arc, and backarc spreading centers to estimate the amount of massive sulfide in the easily accessible neovolcanic zones of the global oceans. The total accumulation in these areas is estimated to be on the order of 6 x 10(8) tonnes, containing similar to 3 x 10(7) tonnes of copper and zinc. This is similar to the total discovered copper and zinc in Cenozoic massive sulfide deposits mined on land but is insufficient to satisfy a growing global demand for these metals.