4 ideas
In 1866 ships worldwide report collisions with a huge, fast, phosphorescent 'narwhal', and the US frigate Abraham Lincoln sets out to hunt it with Professor Aronnax, his servant Conseil and the harpooner Ned Land aboard. It is Captain Nemo's electric submarine Nautilus, and the whole panic had come from reading an unknown machine as an unknown animal.
Nemo builds the Nautilus from parts ordered from many different firms under false names, so no one supplier knows what is being made. He powers it with sodium batteries he makes from seawater and feeds his crew entirely from the ocean. This complete independence from land lets him cut every tie to states, laws and markets, and the book treats technical autarky as the material base for leaving society rather than reforming it.
Verne makes an impossible submarine believable by giving it hard numbers: length, displacement, hull thickness, ballast-tank volumes and how its electric plant works, each explained by reference to known physics. Once the reader accepts the specification, the machine feels plausible and the adventure can move freely, with the technical detail doing the work that suspension of disbelief normally has to do.
Late in the voyage an unnamed warship shells the Nautilus. Nemo rams it below the waterline and watches through the saloon window as its crew drowns. Then he goes to his cabin and weeps before the portraits of his murdered wife and children. The episode shows justified grievance turning into mass killing, which appalls Aronnax and pushes the captives to escape during the Maelstrom.