Warning: Spoilers Follow
El Dorado is a hidden location in 80 Days. It is a major part of the Black Rose story arc. El Dorado is a floating city under the control of the Artificers' Guild and kept secret from the rest of the world. Those who do know about it believe that it is some kind of vault to protect the wealth of the Guild from the banks and taxes of other countries.
On the way to Buenos Aires from Santiago, if Passepartout has met and agreed to help her before, the Black Rose will contact him again and inform him that she has obtained information on where the floating vault will be docking next; off the coast of South America near Buenos Aires. To get on board he will need to gain access to the 'upper city', simply by traveling to it and declaring that he is 'Passepartout, valet of Phileas Fogg' or showing the Artificers' Medallion should you have one in your case. If contacted by the Black Rose, she will join them while in the elevator leading upwards, disguised as the Lady Aodha. Once there, the Artificer Elect will interview Passepartout and the Black Rose if she joins them, and eventually the Artificer will allow them passage on the El Dorado, hinting to them about the secrets they may find on board.
A gyrocopter lands in the city and flies Passepartout, Phileas Fogg, and the Black Rose several miles out to sea, where El Dorado is docked. It is a massive complex, a city-ship that is entirely devoid of people and instead crewed solely by automatons who seem to have the capacity to make conversation. It is at this point that the Black Rose reveals her true identity; she is Rose Fogg, illegitimate daughter of Phileas Fogg and a servant of the Reform Club. Though Passepartout is dubious she insists it is the truth, and then immediately sets out to look for the entrance to the hidden vault of the Artificers.
One day, while taking a walk and searching for any clue of a hidden entrance, Passepartout can find an automaton that has been cleanly cut in half and is now missing its legs. Surprisingly, the automaton asks what happened to its legs, since it cannot perform its functions properly without them. Passepartout can try to comfort the automaton but it insists that without its legs it cannot complete its duties and must therefore be recycled; the prospect of getting new legs is out of the question to the automaton, who says that its old legs were just right. If Passepartout asks the automaton to try to remember how this happened it will cryptically say that something is wrong with the ground before its cogs stop turning and it powers down. Investigating the nearby ground will reveal a patch of grass that is actually just metal made to look like grass and is apparently concealing a secret hatch. If he tells Rose of this discovery she will immediately follow him back to the site and attempt to open the hatch, although it will actually do so on its own. Should Passepartout and Fogg have reached El Dorado without Rose, the injured automaton and hatch are both discovered, but the plot goes no further and the island continues the journey to Dakar without further incident.
Down underground it is completely dark after the hatch closes again on its own, and the pair hold hands while attempting to navigate this cavern which seems to stretch underneath the entire span of the city. Eventually they happen upon a figure holding a light which clearly shows that the figure is an automaton. It offers some explanation to the two would-be thieves about El Dorado and this cavern; the treasure that the floating city protects is not the gold and money of the Guild, as Rose believed, but instead the crystal walls of the cavern that the city is built on top of. This natural crystal can be cut, carved, and engraved into control shards that don't just make mindless machines but instead automata with real desires, personalities, and souls. 'Naturals', as these living automatons are called, make up the entire population of El Dorado; indeed, the floating city was built not only to protect this crystal but also to provide the automatons with a safe place to live. The automatons can choose their own occupations and even design, changing things about themselves until they get a model that makes them comfortable. It is unknown how these natural crystals do what they do, even to the Guild which has been studying them for the some time now, but some theorize that the personalities that the crystals embody are the souls of the dead that have been trapped in the crystal walls.
Although at first disappointed by the lack of real treasure to steal, Rose realizes the importance of El Dorado, the crystals, and the value of this place remaining a secret from the rest of the world. She makes herself and Passepartout swear a vow of secrecy to never reveal what they have learned today to the world, to which he absolutely agrees. The automaton opens the hatch from which they entered back up again and sends them on their way. A few miles from the coast of Africa the city docks again and another gyrocopter is sent to pick up the three humans and fly them to nearby Dakar. Once there Rose will reveal that she may or may not be Fogg's daughter, as she only has her mother's word to go on, and that she will be laying low for a while. She asks Passepartout to come with her and live a life of freedom and adventure, to travel the world without the constraints of a wager holding them back. At this point Passepartout can either choose to decline her offer and finish the wager with his master, or accept and let Fogg return to London alone. If he decides to go with her a number of possible routes all over the globe will open up for the two to choose from. After choosing one, the two will start their life of adventure together and the game will end.