In this world, I haven't visited the End or the Nether yet, so the
/DIM-1 folders have both been empty.
At some point today, after building a long rail line, I noticed that there is now a 12kb
/region/r.1.1.mca in both folders, even though I haven't been to either dimension! The files are missing in my backup from yesterday, so I know they were just created.
This is a single-player map, so that precludes the possibility of another server user having visited and caused chunks to be generated.
Where did they come from? Why is Minecraft generating chunks for dimensions I haven't visited?