I know I'm just nitpicking but "undersky" is actually more correct. german language has a thing where you combine different words to create a new word. unterhimmel doesn't refer to something that is placed under the sky, but it refers to "The Undersky" itself. depending on your interpretation, it could be a metaphor for hell, since it's the sky that is bellow/ upside down. or it could simply refer to the place that is literally positioned under the sky.
Nice interpretation, I´m from germany myself and I actually didn´t came to the idea that Unterhimmel could be a metaphor for hell, but that´s a pretty cool thought! I myself rather thought that the Unterhimmel rather is a lower layer of Heaven since Himmel is not only sky but also heaven and Taishi references to angels/heaven/etc. a lot owo I came to the conclusion that this might be a possibility because the opposite to "Unterhimmel" would rather be "Oberhimmel", with the translation "upper heaven" than "Überhimmel" (over heaven/ over sky), which would imply that Unterhimmel is Undersky. Welp, I hope what I´m saying makes sense, sometimes my english isn´t quite fluent :eyes: