Well, I'm basically writing a fanfic, and I wanted to know if/how a messenger butterfly could get from Chireiden to the Netherworld, and if doing so requires it to pass through Gensoukyou itself. Also, I wanted to know how hard it is for a living mortal to pass through the barrier around it that SHOULD prevent living people from entering (And completely prevents those without death like Mokou from entering). Reimu, Marisa and Sakuya had no problem entering in PCB, but...
Der. *Headdesk* Yukari weakened the barrier. Okay, so I'll assume that any youkai with sufficient power can enter the Netherworld?
At the moment, I'm thinking Gensoukyou exists in 'layers'. So the Underground is under Gensoukyou but just above Hell, while the Netherworld is located above Gensoukyou but under Heaven (Or wherever it is Celestials come from). Makai would be somewhere between Hell and the Underground, and there're barriers/borders/whatnot preventing accidental discovery or entrance (The Sanzu river for Hell, the vents for the Underground, the barrier for the Netherworld etc.). How accurate would this model be?