*update: There's another article on the Discover Magazine Blog that explains this even better. The 800,000 square mile shelf was above sea level in the last ice age. (Just to be clear, the main mechanism for getting water out of the ocean is to pile it up on the continents in the form of snow and ice in glaciers. Ice age = low sea level.) Anyway, the East Siberian Arctic shelf was out of the water and vegetation grew there, absorbing carbon dioxide. (You may ask, if the earth was so cold and covered in glaciers, how did plants grow in Siberia? Well they did it very quickly in the long days of summer. This is the low lands. The glaciers would be in the higher elevations.) When the ice age reversed and the sea level came up, the shelf became submerged before the organic matter was completely decomposed. It was prevented for breaking down entirely by a layer of ice in the form of permafrost. Now that barrier is breaking down and the decomposition is picking up again, releasing methane into the water.
I don't really have anything to add. I worked in the yard today, digging up more conduit. I pulled the wire completely out of the foundation of the old house. It was easier than I thought it would be. It was about like pulling a garden hose through wet grass. I'm pondering what to do with it now.