There was a documentary sometime ago about one which hit the whole of the United States Eastern seaboard thousands of years ago. The evidence is still there such as vegetation and tree lines showing how high the water went.
It was caused by huge sections of the Canary islands falling into the sea.
It will happen again and the likely candidate is more than half the island of Lanzarote collapsing into the sea. It's estimated it will cause a Tsunami more than a thousand miles in length, 200 feet high and travelling at supersonic speed by the time it reaches Manhatten island for example..