Undergrounder (アンダーグラウンダー, Andāguraundā) is the 19th chapter of Clockwork Planet manga series.
After Naoto and RyuZU emerge from the underground, Marie kicks him in frustration. He tells her of his expeditions for the past 20 hours.
Naoto and RyuZU land about 20,000 meters below the room where they fought AnchoR. She tells him that normally this location would be an uninhabitable mine that would guarantee Naoto's death within 10 seconds, but a core tower is nearby that must be controlling the climate, albeit with limited oxygen. They meet a man, who reveals himself to be a retired engineer, in order to ask him to find a way up. The man commends RyuZU for looking after Naoto, and comments that she must be an Initial-Y series, seeing as how no ordinary automata would have her scythes and be able to survive such a massive drop. The old man begins to ramble about Y and his mystifying invention of the Clockwork Planet, no scientist which had ever deciphered the secret to and which inner workings elude the greatest engineers to date, including the old man himself. He offers Naoto to repair the elevator to the surface if he answers a question - considering all of mankind's suffering after every challenge, does he feel any doubt towards the world? Naoto defiantly exclaims that just because he lost all hope doesn't give him the right to represent humanity.
Marie and Vainney tell Naoto of the situation involving the massive mobile weapon. As Marie continues brooding on the hopelessness of the situation, Naoto begins to insult her, inspiring her to find a solution as a woman who thinks nothing is impossible. She asks Naoto if he's willing to lose his name and become a terrorist, to which he accepts. They plan to save the world and AnchoR, and attack Tokyo before Mie does.
As Naoto was leaving the underground via elevator, the old man concluded that the young boy must be Y.