At the heart of a buried prehistoric timber henge in England is a giant tree. But not just any tree: this one is upside down. And growing.

Working as an artist for a secretive archaeological dig just outside his village, teenager Robert Drew is about to have his world turned upside down. A series of strange events begins to unfold when he meets his bratty little sister Chloe on his way home one day. Nothing too extraordinary -- except Chloe lies comatose in a nursing home after a riding accident three months ago.

Meanwhile, back at the henge, muddied birds are struggling up through the roots of the emerging tree. And then Rob meets Vetch, a self-claimed druid and poet who tells him that Chloe is trapped in the Unworld beneath the henge, and that her coma is the result of an encounter with one of the Unworld's denizens. If she has neither eaten nor drunk anything of the Unworld, she may still be rescued. Rob initially dismisses it all as new-age twaddle and suspects Vetch is trying to manipulate him. But despite his skepticism, he is slowly drawn into a world of myth and mystery -- the greatest of which is entirely human.

British author Catherine Fisher cooks up an enticing, if not entirely successful, mixture of Welsh mythology, archaeology and contemporary family drama in this young adult fantasy. Building up steadily from rather prosaic beginnings, Darkhenge gradually reveals all its disparate elements to be unexpectedly connected. It's hard to say much about the Unworld without exposing some of its mysteries, but the story of Taliesin and Ceredwen, the capacities of the human imagination and sibling rivalry are all involved.

Fisher writes vividly and knowledgeably about her many subjects, but the book's characters, while often interesting and sometimes intriguing, seem to lack any profound emotional connection with each other -- a connection that proves absolutely vital to the book's central plot. For all the anger and resentment the various characters are supposed to feel, true sparks never fly between any of them.

Unlike the henge of its title, Darkhenge feels a little hollow at the center of all its ideas and action. It's an atmospheric, absorbing read: quickly read and, apart from a few striking images, as quickly forgotten. For a richer and more satisfying treatment of some of the same themes, John Connolly's The Book of Lost Things is a better choice.