Has anyone ever gone back? Did it just disappear? Why did the original man leave Atmora in the first place? Is there anyone else still there? Same for the Redguard native land, what of them?

No it froze, to the level of inhospitability (I think, definetely became extremely cold anyway, perhaps some Nords remained). Yokuda (Reguard homeland) sank ages ago.

http://elderscrolls.wikia.com/wiki/Atmora

That should answer your questions.

Very, very interesting. I did not know Tiber Septim came from Atmora at all.

its in the same place waiting for that the Devs team do some work on it!

I think only one or two books obscurely mention it in passing. Prior to talos, the rulers of the first empire were at best, quite savage and violent and unable to unite everyone.

Why do people link the crappy wikia?

UESP, mother [censored]. Do you know it?

http://www.uesp.net/wiki/Main_Page