Not P but Moby-Dick (77)

[Mike Jing]

>From Chapter 99:

Ere now it has been related how Ahab was wont to pace his quarter-deck,
taking regular turns at either limit, the binnacle and mainmast; but in the
multiplicity of other things requiring narration it has not been added how
that sometimes in these walks, when most plunged in his mood, he was wont
to pause in turn at each spot, and stand there strangely eyeing the
particular object before him.

What does "plunged in his mood" mean here?