Naughty school kids once used to hurl glass vials that launched the most pervading smell of rotten eggs when they smashed.  Stink bombs produce hydrogen sulfide.  Interestingly, if you can smell it you are more or less safe – though not from flying glass shards.  When H₂S is more concentrated, it becomes an odourless and stealthy killer, as ‘sour gas’ emitted from oil drilling rigs.  A group of anaerobic bacteria generate the gas when there are abundant sulfate ions in oxygen-starved conditions.  They use these ions as electron acceptors in their metabolism, thereby reducing sulfate to sulfide ions; a common phenomenon in stagnant swamps, and especially prevalent at depth in the Black Sea.

Several times during the Phanerozoic global ocean depths became anoxic, when thermohaline circulation shut down.  The consequences show up in black mudrocks, rich in partially broken down hydrocarbons and iron sulfide.  Some of these are major source rocks for petroleum.  Unstirred by deep current flow, bottom waters pervaded by H₂S are covered by oxygenated water, so it might seem that there is little threat to surface dwellers and air breathers, although any animal unwarily entering toxic bottom water would instantly die.  That is why black mudrocks are repositories of exquisite fossils.  Should H₂S build up in deep water, however, there might be chemical instability that would result in large-scale emissions to the upper ocean and to the atmosphere.  Geochemists from the universities of Pennsylvania and Colorado have made some simple chemical calculations to see if such a potentially catastrophic leakage is within the bounds of possibility (Kump, L.R. et al. 2005.  Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia.  Geology, v. 33, p. 397-400).  Theoretically it is, once a threshold concentration of around 1 mmol kg^-1 of H₂S dissolved in deep water is exceeded.  There would be sulfidic upwellings involving emissions of the order of teratonnes of sulfide per year to the atmosphere; more than 2000 times that today from volcanoes, with the added risk that it would also permeate upper-ocean water.

As well as witnessing mass extinctions, the Late Devonian, end-Permian and Middle Cretaceous were characterized by widespread anoxia.  Leakage of H₂S would not only have killed directly, but would have destroyed the ozone layer that protects from UV radiation.  Inevitably, methane produced by other anaerobic bacteria would also have been released in the same way to force global warming.  Rather than being the result of dramatic impacts or monstrous flood basalt effusions, mass extinctions at these times would have been quiet, but efficient nonetheless