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Submarine caves, cavities and niches characterised by H2S elevated concentrations are particularly interesting for their inhabiting microflora as well as for the overall chemical, geological and biological parameters. These ecosystems are usually populated by well adapted living forms, physically distributed following the in situ concentration and gradient of micronutrients, O2 and H2S, and also according to the values of temperature and pH. The biota is primarily characterised by prokaryotes (both autotrophic and heterotrophic) adapted to anoxic and/or microaerophilic condition and capable to form extensive biofilms on the rocky surfaces and even on the bottom sediment. These habitats can be defined as extreme, because the scarcity or absence of solar irradiation, the chemo-physical traits and the fact that specialised prokaryotes are often the only inhabitants. This review is focused on the microbial ecology of marine caves and holes characterised by high levels of hydrogen sulphide. Ecological and geological data are already available but very few insights as far as regard microbiology were achieved in order to describe these fascinating habitats. The autochthonous mesophilic and thermotolerant microorganisms living in these caves may have interesting physiological traits and eventually may lead to potential application in biotechnological processes. [ABSTRACT FROM AUTHOR]