My attempt at summarizing:

There's a relatively shallow part of the North Sea between Great Britain and continental Europe. Some of this area was above sea level during ice ages, and that area is called Doggerland. It's believed to have been a full land bridge at one point, but it became islands as the sea level changed. Its largest island, referred to as Mesolithic Doggerland (MD), might have been inhabited until about 8000 years ago. Land slides in Norway probably caused tsunamis, including a major one called the Storegga tsunami. The tsunamis could have wiped out the inhabitants there or at least led to their decline.

The Doggerland entry on Wikipedia suggests that it isn't fully known how quickly the sea level changed at times.