Acceptable Targets: The British, the author being a Frenchman. Somewhat ironic because Verne was far more pro-British than most of his countrymen (through that might not have been saying much, with The Napoleonic Wars in living memory at the time). And before them, it was the Russians. Verne wanted it to be the Russians, but his publisher balked at the idea, since France was on good terms with them, at the time. Not only that, but the sales as well. Russia always was a huge market for Verne's books, at one point being the largest of them all, and Hetzel didn't want to lose all that money by offending and alienating the readers. He managed to persuade Verne to change Nemo's nation, and he, ever the pragmaticnote Perhaps surprisingly, but he was. In his youth, when his father withdrew his financial support after learning that his son was dabbling with literature instead of studying the law, Verne had to earn his living by being a stock broker. He hated it with passion, but was quite successful, becoming financially independent even before breaking out in the writing field., agreed.
Ass Pull: Whilst in the Maelstrom, the protagonist writes about how no ship has ever escaped the vortex. He is knocked unconscious, and when he comes to, he and his friends are safe, and offers no explanation as to how that was achieved.
— "What happened that night, how the skiff escaped from the Maelstroms fearsome eddies, how Ned Land, Conseil, and I got out of that whirlpool, Im unable to say. But when I regained consciousness, I was lying in a fishermans hut on one of the Lofoten Islands. My two companions, safe and sound, were at my bedside clasping my hands."
The title refers to the distance that the Nautilus travels while under the sea, not the depth that it dives to (20,000 leagues is actually about twice the circumference of the Earth).
Captain Nemo is the antagonist of the novel, not the protagonist. Though he's certainly the most famous character in the novel, he's an Antihero at best, and a full-on villain at worst. This misperception is probably because Aronnax is the Unreliable Narrator who idolizes Nemo before he fully understands what's happening.
The Nautilus crew didn't have an epic showdown with a giant squid, they had a prolonged battle with an entire school of giant squid. The Disney film contributes a lot to this misconception, since (presumably) the studio only had enough money in the budget for one giant animatronic squid.
Related to the above: it's somewhat debatable whether Verne actually meant his famous monsters to be squid; in the original French text, he referred to them as "poulpes" ("octopuses") rather than "calmars" ("squids"), and many early English translations likewise called them "poulps" (an archaic English term for octopi). This may have been in the interest of greater scientific accuracy: in the years since the book was published, zoologists have become mostly certain that Real Life octopi don't actually grow to the gigantic proportions seen in the book, though Real Life squid do.
The most common image of the Nautilus has her equipped with a huge saw-like raking blade, and this is often said to be Verne's original idea. In fact, this was designed by Harper Goff for the Disney live-action movie: Verne's Nautilus was said to be cigar-shaped and have a ramming prow.
Don't forget that he has something of a Misaimed Fandom purely because of his wonderful toys— you read ocean explorers like Robert Ballard and Jacques Cousteau saying that as children, they read the book over and over and "wanted to be Captain Nemo", meaning they wanted his awesome submarine and diving gear, not that they wanted to be supervillains, and two later real-life submarines were named "Nautilus" in homage.
Well, not really. Nemo desire is just to "severe all ties with mainlands". The two ships damaged at the begining of the book are so by accident, as Nemo claims. Then is implied that he sinks several warships that were chasing him, but only on self-defense. He even funds freedom movements all over the globe. Yes, he is a misanthrope, and yes, he is "not civilized" as he puts it; that doesn´t make him a villain. He only crosses the Moral Event Horizon once, at the end of the novel, and for at least understandable reasons, if not fair reasons. He wants just to be left alone in the fathoms of the ocean.
First Installment Wins: The sequel to the book, The Mysterious Island is actually considered an excellent book in its own right but it's far more obscure than the first novel.
Funny Moments: Conseil hopes Ned Land will get some red meat soon, "lest sir wake up one morning and find only chunks of me to serve him".
A lot of the descriptions of whaling and fishing and the beauty of the sea becomes this due to modern pollution problems, rampant overfishing, and climate change. Worst of all, some species described in the book (the greater Auk, for example) are outright extinct. (Granted, the Auk was extinct then, too.)
Verne accurately predicted the high speed and secret conduct of today's attack subs, and the need to surface frequently for fresh air (with diesel subs). He also saw the danger they presented to the Royal Navy.
Magnificent Bastard: Captain Nemo, born Prince Dakkar, is revealed as the son of an Indian Raj whose family was slaughtered for his rebellion against the British. Nemo has since abandoned civilization to create his own by his brilliance and force of will, resulting in the Nautilus-an ingenious submersible that Nemo travels the world with, striking fear into the hearts of the British by destroying their ships. Nemo is polite even to his prisoners, taking everything they need from the sea and their own victims, while maintaining a air of civility and geniality, showcasing his unmistakable charisma and drive.
The crew spend an entire chapter killing sperm whales for no particular reason, except that Nemo doesn't like them.
Several times they find endangered species, such as a sea otter and a dugong, and kill them. Even when they point out how bad things are going to be if humans don't stop killing and eating endangered animals, they still proceed to do that exact thing with little remorse. And the hunting of a supposed undiscovered species of "giant narwhal" that starts the plot of the book in the first place, as well.
The crew also treat Papuan natives who attack the Nautilus like savages instead of people; they don't even stop one from wandering onto the (electrified) staircase of the ship.
However, also Ned Land, Counseil and Aronnax treat them as savages. Maybe could be said that it was Fair for Its Day? Captain Nemo lampshades that "savages" can be found at any part of the world, and even when the papuans wandered on the electrified staircase, it's stated it was only capable of repelling them and not killing them. Nemo said:
"Well, sir, let them come. I see no reason for hindering them. After all, these Papuans are poor creatures, and I am unwilling that my visit to the island should cost the life of a single one of these wretches."
Writer-Induced Fanon: Nemo's ethnicity as an Indian, a Sikh Prince to be exact, was made explicit in the novel's sequel, The Mysterious Island but was left ambiguous by Verne in the first novel. There are many in-text hints such as when Captain Nemo rescues an Indian sailor and admits that he sees him as belonging to the country of the oppressed and considers him a countryman but it's still quite cryptic. Most adaptations made Nemo an European. Verne did intend him to be Polish originally but on advice by the publisher changed it, but it was Verne's own idea to make him a Non-European.
The 1954 film adaptation contains examples of:
Designated Hero: Ned Land is kind of a jerkass throughout the film, at least to their "host". Though when we see how bad Nemo really is, he seems a little justified.
Designated Villain: Nemo comes cross as this at first — until you see his true colors. Ned seems more than a little justified in mistrusting him after he sinks a ship full of innocent sailors. This can, in large part, be blamed on James Mason's charismatic performance.
Got a whale of a tale to tell ye, lads A whale of a tale or two 'Bout the flappin' fish and the girls I've loved On nights like this with the moon above Whale of a tale and it's all true I swear by my tattoo!
Hilarious in Hindsight: The dishes which thoroughly revolted Ned Land and Conseil (and also 1954 American audiences) at dinner, are routinely found today in most Japanese and other East Asian restaurants everywhere; i.e. sea snake is eel, unborn octopus is either baby octopus or fish eggs, and sea cucumber is still called sea cucumber (but still no whale milk, anywhere so far).
Nightmare Fuel: The sight of the well lit Nautilus speeding towards their ships. Made even scarier when you see the fanatical and hateful glare of Captain Nemo as he rams a ship in one scene.
Older Than They Think: People often forget that there other film adaptations of the book made around fifty years before this one, when the motion picture business was still in its infancy. The earliest one was a silent short film made in 1905, but there were other adaptations made in 1907, 1916, and 1917.
Visual Effects of Awesome: The battle with the giant squid. Even today it looks awesome. Which is kind of funny, as the battle as it was initially shot (in daylight with calm seas) was the inverse of this trope. A little darkness and splashing water to cover the obviousness of the huge puppet, and you have a classic.