I am translating a book into my own language, and I have encountered the sentence like this:
Most of modern Finland still lay beneath the sea, which means that the first settlers must have arrived from the south and the south-east. It cannot be known for certain what language these nomads spoke and expert opinion is still divided on the matter. Some maintain that Finnish, which is not an Indo-European but a Uralic language, has been spoken in Finland for only the last 1,500 years.
— "A History of Finland" / HENRIK MEINANDER/ page 2
I cannot catch the meaning of the first sentence. I add 2 more sentence in order to make context clear. I have 2 variants:
Firstly, I thinks it is related to existence of many lakes in Finland. My second variant is that in the past Finland was called suomi, land of swamps. Is the author implying this?