Beneath the oceans lies more methane than we can ever imagine, frozen under low temperatures and massive pressure. If the oceans warm, however, these methane hydrates will become unstable, return to gaseous form and rush to the surface to cause all sorts of havoc.
Methane is a 22 times more potent greenhouse gas than CO2. Large scale methane hydrate release would almost certainly push the earth to the 6 degrees doomsday scenario and keep it there for centuries.
Methane is also flammable at relatively low concentrations and a massive concentrated release can be ignited by natural phenomena like lightning to explode with the force of several nuclear bombs, vaporizing everything within a radius of several kilometres.
Methane hydrate release is speculated to become an irreversible climate change driver at 5 degrees of warming. For the sake of future generations, let's make sure we never get there.