In all honesty, my first interpretation of this story was a little out in left field, with little to no support from the text. But whatever, this is just what I got from it. The end of the book reminded me of an episode of Torchwood, called "Small Worlds" in which 'fairies', humanoid entities native to Earth, attempt to abduct a young human girl as their 'Chosen One'. Jasmine, said 'Chosen One', is friends with the fairies, but is unable to see them as the terrifying creatures they can truly be (see picture to the left). When Ib returns after disappearing in the tunnel, she tells about how she was playing with 'the girls'. To Ib, these girls seemed just as harmless as the fairies appear to Jasmine. Due to this similarity, I believe that the tunnel might not have been a representation at all, but perhaps a hint that there could be terrifying things in our world that many have yet to notice.
