Does this passage really lead from Kumaso all the way to Izumo?Well, given the orientation of the entrance to the Yamataikoku Underground, and the orientation of the entrance to Seiryuu Grotto within it, I consider at least some portion of Seiryuu Grotto to be located beneath the town of Kumaso.
For the purposes of in-world explanation, then, yes - from Kumaso to Izumo
The hell are you talking about anyway?Oh... well, I discovered that upon completion of the Seiryuu Grotto, as you pass through the final room before exiting the dungeon, one of the paltry "fences" constructed to keep you out of a neighboring tunnel isn't at all adequate to actually keep you out.
Run past it, and you'll discover an enormous cavern.
After exploring the cavern enough, I recognized that on an upper level, it was the boss room at the Cave of Nakisawame, where you fight the Wanyuudou.
Sure enough, it seems that Seiryuu Grotto is "connected" to Nakisawame Cave in Izumo.
http://youtu.be/9SBS8Err2GIWhat secret places have you found?