Birds 'back then' used to be able to pass into other worlds at will, you see.

As the "Bird of the Underworld", I unthinkingly went there on some summer nights, but it's usually the land of the dead... What was I, suicidal?

That's why I could come to the Surface so easily, too.

Birds these days, though...

...So after all, the place she went...

was the Underground?

Soku-no-Kuni is said to be the land below the earth, after all. She should find some entrance in that gloomy place.

I feel it was already in pretty bad shape at the time I went, though...