Siberian Times states that thousands of bulging gas bubbles have been located in Siberia. These are thought to explode to form the giant craters found in the area. Scientist say it's thawing permafrost releasing methane caused by elevated temperatures. Article contains amazing pictures.
Similar observations have been made around the Arctic regions. This is a cause for concern as methane is a potent greenhouse gas creating a positive feedback loop [wikipedia.org], there is potential for a chain reaction.