Some of the Global Warming theories propose that as our average temperature increases each year, parts of the Earth’s surface that were covered by ice and snow are exposed to the atmosphere and sunlight. And organic matter that was trapped in those areas during past ages starts getting released into the atmosphere resulting in even more greenhouse gasses.

Regarding those underwater methane deposits, if I remember correctly, one report that I saw a while back stated that there is enough hydrocarbon fuel in just the deposits located off the West Coast of Canada to provide energy for the entire planet for a while.

When that frozen material that can be a mile or more deep in the ocean is rapidly brought to the surface it reportedly detonates. So, mining those deposits is considered problematic.