It turns out entire continents from previous Elder Scrolls games are hidden beyond Skyrim's borders.
Like a true Nord, when determined explorer Jesse was faced with the invisible wall that marks the edge of Skyrim, he broke out the noclip console command and kept on going. He wandered through a poorly textured wasteland until finally he came across a pair of peculiarly familiar landscapes; Morrowind and Cyrodil from the previous Elder Scrolls games.
The "continents" are merely blurry landmasses with a singular distant texture, though Cyrodil does still have the iconic White Gold tower which dominated the horizon in Oblivion. It's unclear at this point what the landmasses are for, as you can't reach or even see them without the use of the noclip code. Some users have suggested the landmasses served as a scale reference during the construction of the Skyrim region, while others reckon they're there for the benefit of the inevitable wave of modders that will come out of the woodwork when Skyrim's official mod tools are released.