Submarines as we know them didn't exist in 1869. But Jules Verne had an almost eerily prophetic knack for knowing what technology would be used in the future -- and he put it to work in "20,000 Leagues Under The Sea," a slow-moving adventure tale with plenty of proto-steampunk and almost fantastical undersea life. 
 Ships in the middle of the ocean are suddenly seeing -- and being attacked by -- "a long object, spindle-shaped, occasionally phosphorescent, and infinitely larger and more rapid in its movements than a whale." 
 Eventually the US government sends out a ship to capture the object, and during a sea battle Professor Aronnax, his manservant Conseil and harpooner Ned Land go overboard. Soon they're picked up by the Nautilus, the vast submarine that has been causing all this trouble, and introduced to Captain Nemo -- an intelligent, charismatic man who belongs to no nation. 
 Aronnax becomes fascinated by Nemo, his ship and his library -- as well as the amazing underwater adventures that Nemo introduces them to, like pearl-hunting and fighting a giant squid. But the captain's free, lawless life has its dark side, and the three men begin to realize that they must get away from the Nautilus no matter what. 
 It's actually rather amazing that Jules Verne not only dreamed up the idea of a semi-modern submarine long before they existed, but thought out the applications, the stealth, and the vast size. And since nothing like the Nautilus existed at the time, there's a slightly fantastical, steampunk flavor to "20,000 Leagues Under the Sea." 
 And as usual, Verne painstakingly studies everything about his imagined world, filling it with science (although he obviously didn't know about water/air pressure) and lovely descriptions of the Nautilus and the eerie underwater world (giant oysters, forests, Atlantis). The only flaw is that he tends to ramble on about exact measurements and travel details; there are boring patches here and there. 
 But Captain Nemo is probably one of Verne's most fascinating characters -- a charismatic, embittered man who is a sort of noble sea pirate. He does some stuff that is totally unacceptable (sinking a random warship), but he also has little spurts of kindness and generosity towards poor and powerless peoples of the world. He's scary but fascinating. 
 Giant submarines, charismatic pirates and an undersea world just waiting to be explored -- "20,000 Leagues Under The Sea" is a fascinating sci-fi classic, if you can get past the dull patches.