After the first few chapters of Twenty Thousand Leagues build up the power and magnitude of the sea creature—presumed to be a massive narwhal—wreaking so much havoc, it turns out to be a submarine. This is an example of situational irony because characters and readers both anticipate the monster to be a living, breathing being but, in a plot twist, it is actually a man-made machine.
Aronnax’s descriptions of the creature before he knows it’s a submarine add to the irony, as he compares the creature to a machine (foreshadowing the big reveal):
Notwithstanding the distance, and the noise of the wind and sea, one heard distinctly the loud strokes of the animal’s tail, and even its panting breath. It seemed that, at the moment that the enormous narwhal had come to take breath at the surface of the water, the air was engulfed in its lungs, like the steam in the vast cylinders of a machine of two thousand horse-power.
While Aronnax never goes on to share the actual horse-power of the Nautilus, his fanciful description of the creature as a machine does capture the might of the submarine that he will soon get to know. The irony, of course, is that he had no idea such a machine could even exist since, until that point, Captain Nemo kept his innovations to himself.
