Caves, like the Survivors sculpture, are another form of symbolism in this novel. As the sculpture emerges and submerges according to the tide, the ebb and flow of the water, so the characters and their thoughts surface and disappear into and out of the caves. It is dark inside, footing is unstable, and this represents not only the events that occur within them but also the deeper thoughts and feelings of characters while underground. I thought Harper's use of the caves in this novel was brilliant. In her last book she used the stark naked, nearly inescapable sun to create, so effectively, a disquieting atmosphere, and in this work she works with the opposite direction: underground, inside, among shadows. Yet she is just as successful using the caves and her characters within them to create amazing tension and anxiety not only for them but for her readers as well. There are spaces of light within, of course, and these reflect moments of thought or insight - some positive, some not so much so. There are references to Kieran and Olivia's meeting place in the caves as being higher up and references to crossroads of sorts within the caves - alluding to choices characters must make. The use of the caves is really almost a story in itself.