UPDATE (2 December): A couple of readers have pointed out that the title is a play on a line from Coleridge's "Kubla Khan":
In Xanadu did Kubla Khan
A stately pleasure-dome decree:
Where Alph, the sacred river, ran
Through caverns measureless to man
Down to a sunless sea.
No one seems quite sure what the allusion means though. Maybe an example of slippage of meaning between oral and written versions of a text? Perhaps Ed will enlighten us in due course.
UPDATE (3 December): More on "Ralph" here.