Theorists are divided about the cause of the Permian mass extinctions. Some hypothesize that the impact of a massive asteroid caused a sudden disappearance of species. However, a look at the carbon-isotope record suggests that existing plant communities were struck down and re-formed several times. To produce such a pattern would require a succession of asteroid strikes thousands of years apart. Other theorists have proposed that volcanic explosions raised the C02 levels, leading to intense global warming. One problem with this theory is that it cannot explain the massive marine extinctions at the end of the Permian period. A new theory posits that rising concentrations of toxic hydrogen sulfide in the world's oceans plus gradual oxygen depletions in the surface waters caused the extinctions. Fortunately, this theory is testable. If true, oceanic sediments from the Permian period would yield chemical evidence of a rise in hydrogen sulfide-consuming bacteria.