One of the most important technology in the story was the Nautilus AKA the submarine used as the transportation for the crew in the story and harpoons to attack the monster and kill it. Captain Nemo is the one leading the crew and does not like war, but still kills many monsters with the harpoon. Deep sea exploration was possible during the time period talked about in the story submarines were used for underwater exploration, but there was no mythical creatures such as a Kraken or a gigantic squid back then and even today.
The underwater Atlantic ocean was used as the main setting in the story to set the mood of silence and eeriness to the protagonist of the submarine and the land like Ireland and Africa was