Dramatic irony occurs when readers of a story know something that the characters do not.

In "By the Waters of Babylon," we realize when John takes his journey to the Place of the Gods that he has come to the ruins of New York City. Clues include its island location, its many tall towers, a shattered statue of "ASHING" representing George Washington, and the "great caves and tunnels" of a subway system.

John realizes from his exploration that the Place of the Gods was actually built by humans, not gods. This is a life changing revelation for him. He recognizes that if humans built this great civilization, his people can, eventually, rebuild it.

The guesses John makes about how people lived before civilization collapsed tell us that loss of knowledge can be profound. John knows nothing of US history or of the advanced technology that is familiar to his readers. He doesn't know about running water, electricity, or central heating. The concept of trains or subways is alien to him. He doesn't recognize elevators—the bronze doors with no handles—and he has never seen a painting like the one with flowers that he finds on a wall. John lives in a stone age culture that scavenges for metal from the old civilization to survive.

The story points to the importance of handing down knowledge across generations. Once it is lost, it is difficult to regain.