What metaphors are used in Holy Blood, Holy Grail by Michael Baigent?

In Part II, Chapter VI, the choose to discuss the grand master Rene D'Anjou. Among his many accomplishments, D'Anjou is noted here for his creation of the metaphor of the underground stream that the authors believe may denote a way of thinking, such as Gnostic thought, or of a secret bloodline the Priory of Sion protects. This latter idea often appears in authors' research, and serves to foreshadow the revelation of a secret later in the book.