A heavy sea with large waves. Although they are still ice-bound, the
sailors can hear the sound of waves breaking in the distance and know that
the ice surrounding them will soon crack. The novel will return to this
point, as at the very end of his narrative Victor Frankenstein recounts
the same phenomenon from his dangerously vulnerable position on the ice in
3.7.9 and note. See also 3.7.7 and note.