What were the caves and tunnels where john thinks the gods kept their slaves in by the waters of Babylon?

In "By the Waters of Babylon," the caves and tunnels that John imagines as places where the gods kept their slaves symbolize the remnants of a lost civilization. He perceives these underground spaces as a reflection of the oppressive structures of the past, where subjugation and control were maintained. John's thoughts about these places underscore his quest for knowledge and understanding of the ancient world, as well as the dangers associated with uncovering the truths of history. Ultimately, they serve as a metaphor for both the mysteries of the past and the consequences of power.