Atlantis is located deep in the Atlantic Ocean, founded by a freed slave titled as the First. It is not the fabled lost city of ancient myth, but rather a refuge of escaped slaves inspired from the mythical continent. Its citizens have undergone extensive body modifications to allow them to survive under the ocean, like scales and breathing masks which make them look like mermaids.
At Rio de Janeiro, Passepartout and Fogg get aboard a cargo ship to Freetown, carrying mysterious cargo. While in the middle of Atlantic Ocean, the ship was attacked and sunk by a group of mysterious pirates, who drag them deeper and deeper into the ocean, into the hidden city of Atlantis.
There, Passepartout learned that the ship that they boarded was in fact a slave trader ship. And the pirates attacked the ship in order to free the slaves and bring them into Atlantis.