This reminds me of The Swarm [1], a fiction book that starts with freak events happening around the world's oceans. The first of them is about a species of marine worms that, together with bacteria, destabilise the methane clathrate in the continental shelf. This causes the continental slope to collapse, creating a tsunami that hits the North Sea's coasts and kills millions.

[1] - https://en.wikipedia.org/wiki/The_Swarm_(Sch%C3%A4tzing_nove...