[MHGinTN]

An under water landslide can release a large volume of methane gas trapped beneath a hydrate layer. The 'bubble' would surface and create a pretty good wave, with size of the wave depending on the size of the bubble that surfaced. The event might happen in a staggered pattern on the sea floor and result in an additive energy effect to create a massive wave. Once a wave generates, it moves unabated over the surface of the ocean unless it interacts with an object (like an island or another wave system or a current whorl).

[Young Werther]

So what you're saying is:

The sea FARTED!