As noted in this post (via here), Skyrim contains much of the rest of the continent of Tamriel in rudimentary form outside its mountains. bBorderRegionsEnabled=0 in Skyrim.ini and no-clip on does the trick. Thanks, commenters. There’s a huge heightmap out there, well beyond view distance, down into Morrowind, Cyrodiil, and Hammerfall. It’s a bit buggy, mind and seems unstable once you’re out of skyrim. Looks like it’s just a chunk of the world rendered around Skyrim for view-distance authenticity, rather than any possible expansion.