Holy crap. An intrepid Elder Scrolls player, curious as to why the game stopped him at the border between Skyrim and Morrowind (the playable landmass of the third game in the series), engaged noclip mode and decided to look beyond. What he claims he found was... the entire continent of Tamriel.
The whole damn thing.
He took snaps along the way, recording landmarks such as the volcano Vvardenfell and the Imperial City in Cyrodiil (centre of the fourth Elder Scrolls game), and while they're of reduced scale and greatly reduced texture quality (they're basically just lightly textured blobs of landscape), they're there in the game, every time you load it up.
Begging the question... why?
Could be to make your in-game maps and aerial sightseeing more realistic. They could have been used as reference when building Skyrim. Or they could be there as a modder's playground. Who knows!
If you want to try it out yourself, there's full directions on how to do it at the link below.