At the end of the Cretaceous period, about 65 million years ago, huge amounts of methane generated by rotting vegetation lay trapped in sediments more than 500 metres below sea level. At these depths, low temperature and high pressure allow methane to combine with water to form solid methane hydrates.

Enormous quantities of methane hydrate are trapped below the Earth's surface today, and several countries, including the US, Germany and Japan, are interested in exploiting their potential as fossil fuels.

But Burton Hurdle of the Naval Research Laboratory in Washington DC and his colleagues believe methane hydrate reserves have already played a pivotal role in the Earth's history--by speeding the extinction of the dinosaurs. They say huge shock waves generated as the asteroid or comet collided with the ocean floor would have travelled round the planet and freed vast quantities of trapped methane.

Lightning bursts in the disturbed atmosphere would have ignited the methane-rich air. "The atmosphere itself would have been on fire," Burton and his colleagues conclude in a new paper. "This could have contributed to the demise of the dinosaurs," Burton told New Scientist.

As further evidence, the researchers point to an earlier discovery of disruption in late Cretaceous sediments, possibly due to methane release, at Black Ridge off the coast of Florida. More recent activity on the sea floor suggests that trapped methane periodically escapes even in the absence of asteroid strikes. Oceanographers believe a smaller "blow-out" occurred in an area in the Gulf of Mexico measuring 36 by 22 kilometres during the late Pleistocene epoch.

Dick Norris, a palaeobiologist at Woods Hole Oceanographic Institution in Massachusetts, who helped to discover the Black Ridge feature, agrees that lots of methane could have been released from that site, had it been trapped in the form of methane hydrate. He adds that the increased preponderance of the isotope carbon-12 over carbon-13 immediately after the impact also suggests that lots of methane was burnt. But he says this isotopic pattern is not repeated all over the globe.

Other experts are similarly cautious. "I think this idea is very intriguing," says Peter Schultz of Brown University in Providence, Rhode Island, who has studied the late Cretaceous impact. "But I'm not sure even an impact this big would have liberated that amount of methane."

Nevertheless, even local firestorms could have contributed to the dinosaurs' demise. Angela Milner, head of vertebrate palaeontology at the Natural History Museum in London, says that many dinosaurs appear to have been in serious decline before the impact. But she agrees that huge methane fires "could have been the final straw" for some species.