Is it true that the stormy and dangerous North Sea between Britain and Norway was dry land similar to mainland Europe and home to large groups of people as recently as a few thousand years ago?

Answer 1

The area to which you refer is known as "Doggerland". It was the land that connected modern Britain to continental Europe. The landscape was varied, but would've been a hilly marshland with forested areas too. Doggerland started to flood around 6500BC due to climate change & the consequent melting of the glaciers. This forced the hunter-gatherer groups inhabiting the area to move to higher grounds. There is plenty of archaeological & geological evidence that supports this interpretation but the dates might be slightly off, as is often the case with trying to pinpoint dates in archaeology, but close enough to be a good estimate. If you want a quick reference for this area, a good place to start is:

https://www.nationalgeographic.org/maps/doggerland/

Hope this helps!