Mauritania is home the Richat Structure in Africa, a mysterious geologic ringed structure that some think represents the remains of the ancient City of Atlantis. The wonders of Google earth show Muritania as a vast ancient alluvial flood plain which at some point emanated from some site either in ancient Africa or the Mediterranean.
Anyway, it is a generally dangerous, murderous and violent place, so any immigrants aren't exactly going to be what you want camping out across the street from your children's schools.
So the Atlanteans pissed off another alien race, who then used collapsed antimatter (CAM) to sink the entire Atlantean civilization into the earth. Perhaps they were getting too advanced? Time for a restart.
I think CAM was first used by Iain Banks in his Culture Novels. Best galaxy-building SF bar none.
I think the only place we DONT get mass migration from is that Island in the Indian Ocean where they spear everybody who lands on the beach.