The climax is the moment of greatest tension in a story. In 20,000 Leagues under the Sea, the climax takes place when Captain Nemo takes the crazy decision to sink the ship after it comes under attack by an unidentified warship. Nemo seems completely oblivious to the loss of life that this will cause.
No wonder, then, that Aronnax and the other men make the decision to escape from the ship, and even more importantly, to escape from the borderline insane, egomaniacal Nemo, whose very presence threatens their existence. The tension builds as we wonder how—and indeed, if—the men are going to be able to escape. On the face of it, things don't seem at all hopeful. The situation's pretty desperate, what with Aronnax and his men caught between the demented Nemo on the one hand and the huge warship on the other.