Methane hydrates are fascinating; if you're unfamiliar with them, glance at the Wiki entry.
Now, a report in the Independent suggests that the ones in the Arctic are starting to sublimate (converting from solid to gaseous form) and that in some places the Arctic Ocean is "boiling" with methane gas.
Methane is about 20 times more powerful as a greenhouse gas than carbon dioxide and many scientists fear that its release could accelerate global warming in a giant positive feedback where more atmospheric methane causes higher temperatures, leading to further permafrost melting and the release of yet more methane.
The amount of methane stored beneath the Arctic is calculated to be greater than the total amount of carbon locked up in global coal reserves so there is intense interest in the stability of these deposits as the region warms at a faster rate than other places on earth.
It doesn't matter if you think global warming is happening or not, or whether you think humans are causing it or not. If the "Clathrate gun" is "going off," it could theoretically trigger extinction events within a human lifetime. Certain religious groups might eagerly anticipate such a event, but personally I'd rather give it a pass.