Let's assume a human civilization have been living for centuries, nay millennia, in a very large cave, not too deep under Antarctica - deep enough not to freeze, but shallow enough not to die because of underground heat.
In fact, unlike some other fictional underground civilizations, this one is aware of the surface's existence but all its people know about it is that it is extremely cold, hostile and deserted - well, it's Antarctica. They don't know there exist habitable lands and other humans on the surface elsewhere.
Let's ignore all sustainability questions, i.e. food, power and such.
My question is : How did this civilization possibly get there ?
My first idea was they arrived before Antarctica became the frozen continent we know, but a little bit of research taught me this was long before humans appeared.
My second idea was their ancestors arrived through galleries networks, maybe from South America (closest habitable land). However these tunnels, or at least a big part of them, must be under the ocean, therefore very, very deep (ocean depth + caves depth) and then probably impossible to stay in for long enough to travel for at least 1000km - whatever the mean of transportation used.
I assume they couldn't have arrived simply by boat, because they would immediately have died of cold, if not before, once in Antarctica. Also, the ancestors of this civilization were probably not technologically advanced - unless they were from somewhere like Atlantis, which is not what I want to.
I have been thinking about it and doing research for some time now but can't find any other way these people could have happened to live there. Any ideas?
edit : Another point I forgot to mention is that this civilization stays nowadays yet undiscovered, so they are "hidden" well enough for the explorers not stumbling upon their caves.
English is not my mother language. I apologize for any mistake.