A story of love, more than anything - of Mira’s love for Leah leading to her ultimately releasing her and of Leah’s love for Mira being all she can recall. Beautiful themes within the book, underpinned by water (in various forms) as the books central metaphor - it’s of us, we’re from it. The mystery of the deep remains so; Leah’s transformation is not wholly explained. In both cases, that does not detract from the story’s success, because we’re prepared for this throughout by references to the unknown deep - we still don’t know, but we’ve seen one impact of it.