It was the area that is now the North Sea, and it got catastrophically flooded around 6150 BCE, after it was already steadily sinking because of rising sea levels and post glacial isostatic adjustment.
Why is it a mystery to me?
Because it was something close to a post-glacial North European paradise, it was densely populated, and because no recognizable myths have survived of its final submergence (caused by a huge tsunami that may have lasted for 2 days on end).
And the Strait of Dover - formed right after the tsunami caused by the Storegga Slide - looked quite differently : many huge white cliffs standing close together and by that creating a dangerous situation (very strong sea currents) for those who wanted to sail through it (and they did have boats back then, 8150 BP).
If there ever had been real "Pillars of Hercules", then these would have been it, 8150 years ago.
And those brave enough to pass those huge white chalk pillars would enter a shallow sea with mud shoals and a few remaining islands.
Edited by Abramelin, 11 June 2013 - 06:16 PM.