Marine Biology and the Undersea World
Despite the machinations going on aboard the Nautilus, and particularly within its captain, the main theme of the book is the wonder of the under sea world that we still no very little about. It holds different fascinations for each of the men; Aronnax is a renowned marine biologist, and so the ecosystem that exists under the sea, absorbs him to such an extent that he is able to divert his attention from the threatening situation with Captain Nemo on the submarine. Consell, as Aronnax's loyal servant, is also interested in the marine biology aspect of their adventure; however, Ned Land whose interest is less in conserving the environment under the ocean and more in harpooning it, does not find more than a passing and tourist-like interest in the wonders of the undersea world. Captain Nemo is extremely interested in research and marine biology, as this was one of the reasons he built the Nautilus in the first place, but since the murder of his family he is more attracted to the fact that under the sea there are no boundaries or borders, and they do not answer to any government at all.
The Brilliance of French Men
Jules Verne, a French man, definitely shows a little cultural bias in the writing of this adventure novel. His main character, Aronnax, is French, and renowned across the world for his brilliance and for the quality of the research he produces in the field of marine biology. He is said to be inspired by a real-life marine explorer called Commander Matthew Fontaine Maury, whom Verne mentions several times in the book. Other Frenchmen mentioned in the book include Jean Francois de Galaup, a famous explorer who unfortunately got lost whilst circumnavigating the globe (and who was perhaps the inspiration behind Verne's novel Around The World in Eighty Days), and Dumont D'Urville, an explorer who found the wreckage of Galaup's ship. Their French ancestry is emphasized, and Verne is clearly a proud Frenchman who wants to introduce his countrymen's renown and accomplishments thematically into his book.
Champion of the Underdogs
Nemo does not like civilization and has gone to incredible lengths to exile himself from it. He is shown in the novel to be a great supporter of the disenfranchised and whomever he perceives to be the underdog. He offers assistance to the Greeks who are rebelling against the Ottoman Empire in Crete, and feels sorry for a poor Indian diver seeking pearls without any proper equipment; Nemo gives him a pouch filled with pearls so that he never has to put his life and health in danger again by diving. He considers the pearl diver oppressed because he comes from colonial India.
Futuristic Science
The Nautilus is a very futuristic submarine, acknowledged also by the men who mistake it for a sea monster. It is able to conduct research for itself and to resist attack from any kind of vessel. The breathing equipment used by the divers aboard the Nautilus is also very forward-thinking. It supplies gas on demand - the first of its kind anywhere. The divers are able to exist under the water with this equipment for up to eight hours, which was unheard of at this time.
Revenge
Despite the main themes that revolve around science, and to an extent, science fiction, a human emotion as old as time is one of the key themes of the book; Nemo exacts his revenge on the nation that murdered his family, and despite Aronnax's pleas for him to show mercy, it is hard not to sympathize with a man whose family was shown no mercy by their killers. Nemo also views his self-imposed exile as a kind of revenge because it enables him to roam the earth, going wherever he wants under the ocean, without any constraints or knowledge from the government of any country.
