Hazel had read enough books to know that a line like this one is the line down which your life breaks in two. And you have to think very carefully about whether you want to cross it, because once you do it’s very hard to get back to the world you left behind. And sometimes you break a barrier that no one knew existed, and then everything you knew before crossing the line is gone. But sometimes you have a friend to rescue. And so you take a deep breath and then step over the line and into the darkness ahead.