I just re-read the chapiter where Eva solve the riddle.
She took an atlas (the same as Erika, I guess).
Figure out what is the sweetfish river, and says that a the sweetfish can't swim in the river because it's not a river, but can still swim in the ocean.
This make me think of "a river within the ocean"...And the Ring of Fire seems to be fitting in that case with the
Izu Ogasawara Trench. (And even it could fit with the volcano theory).
Moreover, she says that the entrance is like a gaping wide mouth...sound like a crater.
Trench, Gouge ?