"Things aren’t always what they seem on the surface. Looking at the area between mainland Europe and the eastern coast of Great Britain, you probably wouldn’t guess it had been anything other than a great expanse of ocean water. But roughly 12,000 years ago, as the last major ice age was reaching its end, the area was very different. Instead of the North Sea, the area was a series of gently sloping hills, marshland, heavily wooded valleys, and swampy lagoons: Doggerland." Source: National Geographic.
Note: If I understand this correctly, the Doggerland land mass was already subsiding into what is now the North Sea when the last remaining islands were swamped by a succession of massive underwater landslides called Storegga slides. According to Wikipedia, "The three Storegga Slides are considered to be amongst the largest known landslides. They occurred under water, at the edge of Norway's continental shelf in the Norwegian Sea, approximately 6225–6170 BCE. The collapse involved an estimated 290 km (180 mi) length of coastal shelf, with a total volume of 3,500 km3 (840 cu mi) of debris, which caused a very large tsunami in the North Atlantic Ocean." This ancient cataclysm has led to stories about a North Sea Atlantis.
In recent years, marine archaeologists have made surveys of the submerged landscape of Doggerland. Artefacts such as spearpoints have been discovered and attempts have even been made to recover DNA samples from the seafloor. For example, the Telegraph reported in 2015 that "archaeologists at the University of Bradford have begun a huge project to reconstruct the ancient Mesolithic landscape which is now hidden beneath the waves" and "[s]pecialist survey ships will also take core sediment samples from selected areas of the landscape to extract millions of fragments of DNA from plants and animals which once lived on the lost world."