Manichean mythology taught that five elements dwelt in five dark caverns, from which various parts of the created world emerged. Augustine doesn’t explain these myths in detail, and it’s not important for readers to know the specifics, except to notice that even the poetry and mythology Augustine studied as a child, which he rejected as immoral earlier in
Confessions, is better than what the Manichees taught him. This is probably because, unlike the ancient literature Augustine studied as a child, Manichean myths intentionally trained Augustine to think of God in ways he later rejected as utterly false.