An Atlantean space-ship converted to submarine, Nautilus is Nemo's main weapon against Gargoyle and his minions. Notable features of this ship are: ability to withstand high pressures and depths, exceptionally resistant Space Titanium armoured hull (the clear ports are made from Tektite), and water-jet magnetic propulsion.

The name of the original spaceship was Eritrium, as is discovered by Jean and Nadia in episode 15, while the origin of the Nautilus is explained at the end of the series. Eritrium (sometimes spelled Elitrium or Eltrium) is also the name of a space ship in Gunbuster, and the robot-ship that blast off at the end of Otaku No Video.