There have been large tsunamis caused by underwater landslides - notably the Storegga Slide off Norway that inundated a lot of areas in what are now Scotland and Norway ~8000 years ago.

http://en.wikipedia.org/wiki/Storegga_Slide

The Storegga Slide might also have been, at least partly, responsible for the disappearance of Doggerland under the North Sea:

http://en.wikipedia.org/wiki/Doggerland