What exact locations are covered by Arctic Explorer?

I want the Arctic Explorer achievement but, best I can tell, I've explored every part of every map in Three Horns (divide/valley), The Fridge and Tundra Express.

Are there some other "arctic" maps like Southern Shelf included in this? I'm puzzled as to how I am missing this one. All the maps appear to be 100% filled in unless there are some "named locations" which don't leave gaps in the map when undiscovered or they're on some distant map I haven't checked (I think I have checked all of the maps you can fast travel to.)

Are you positive that you have been everywhere on all 4 maps?

I just unlocked this achievement yesterday. What triggered it for me was a small part of Three Horns Valley, near the southern end of the map that wasn't filled in. I don't remember if it was a named location or not, but I know that once that part of the map was filled in the achievement was unlocked.

I would suggest visiting all four maps, and verifying that all parts of each map are completely filled in.