Scandinavia
The Great Wall of Norway was built by Othar Tryggvassen in an attempt to create a closed environment in which all Sparks could be neatly eliminated [1][2][3]. Whether the creation of the Great Wall has anything to do with the tragic destruction of Oslo has been obscured by the lack of witnesses - Tryggvassen was one of the very few to survive and has apparently not discussed it, leaving it to the confabulators instead.
The Wall and the Crater of Oslo are indicated in red on the small map.