A massive methane explosion frothing out of the world's oceans 250 million years ago caused the Earth's worst mass extinction, claims a US geologist. Similar, smaller-scale events could have happened since, which might explain the Biblical flood, for example, suggests Gregory Ryskin of Northwestern University in Evanston, Illinois1. And they could happen again: "It's a very conjectural idea but it's too important to ignore," says Ryskin. Up to 95% of Earth's marine species disapeared at the end of the Permian period. Some 70% of land species, including plants, insects and vertebrates, also perished. "It's arguably the single most important event...