I really love Diane Fallon as a character. She keeps a calm head under pressure, even when her life and the life of her friends are threatened. Diane has her vulnerabilities, but she does not let them define her.
What keeps this from a higher rating for me is how the "bad guy" comes into the story; he is barely a whisper in the mind of any of the main characters because his existence (even as a police officer, not perp) was revealed so late in the novel. The tension of the climactic caving scenes existed because of the environment, not because we had a good grasp on who he was as a person.