Submarine tailings disposal is conceptually an attractive option for managing reactive mine wastes in coastal regions by virtue of reduced sulfide oxidation under a water cover and no infrastructure to maintain after mine closure. However, the lack of detailed knowledge of metal leaching, dispersal of fine tailings and biological impacts in the marine environment has led to no operating permits currently in effect in Canada. Detailed chemical and ecotoxicological assessments at two historic copper mine sites (Little Bay and Tilt Cove) in the north coast of Newfoundland with tailings disposed in an estuarine to shallow (=100 m) marine environment have demonstrated reduced reactivity of sulfidic tailings when submerged under water. Only non-acute toxicity is detected in biota inhabiting the immediate vicinity (<300 m) of the tailings fallout areas. however, dispersal of fine tailings could not be fully delineated at the higher-energy site (tilt cove). a review of another historic shallow marine tailings operation in the southern tip of vancouver island (jordon river) with a high-energy setting also reveals problems with unconfined tailings dispersal and temporal biological impacts. documented studies at two deeper (140 - 360 m) marine tailings disposal operations at the northern tip of vancouver island (island copper) and northwestern coast of british columbia (kitsault) again illustrate imperfect predictions of tailings re-suspension and dispersal. at these sites, bioaccumulation of tailings-derived metals appears to be minimal. there is evidence of rapid re-colonisation of the tailings surface by benthos but of different species than the original population. integrating these observations with reported metal fluxes and biological diversity associated with submarine hydrothermal vents, the risks and benefits of deep-sea (>1 km) tailings disposal are discussed. To determine if submarine tailings disposal in deep, confined basins on the seafloor is environmentally friendly, detailed multi-disciplinary research incorporating physical, chemical, biological and toxicological components will be indispensable.