This extinction seems to have been caused by a massive series of volcanic eruptions in Siberia, Russia. Enough lava erupted to cover the whole globe in a 20-foot layer. The volcanoes would have also spewed out billions of tons of poisonous gases [1]. As if that wasn't enough, very recently a huge new crater has been discovered in Australia, the second largest direct hit from an asteroid or comet in the last billion years. This could have occurred around 250 million years ago and may have been the trigger for the devastation suffered by life on Earth at that time [3].

It's likely that the volcano eruptions and possible asteroid hits triggered rapid global warming (like today, but for different reasons) which may in turn have set off something much more scary that people are only just beginning to discover about.
This warming may have unfrozen strange stuff called gas hydrate which normally sits on the bottom of oceans. If this stuff is uncorked by warming, it fizzes and melts and blasts off lots of gas called methane. Methane is a very strong greenhouse gas when it gets into the air. And today, there are billions of tons of this stuff lying on the bottom of the oceans... and the oceans are warming up! [2]. If you want to find out more about greenhouse gases and global warming, take a look at my Global Warming Guide.

1. Science News, 16/3/96, 164
2. Science News, 16/5/98, 308
3. New Scientist, 29/4/00, 19