Obtain the Key of Serpentis, then play level 10-7, "Jokulkyrkja", in Antarctica. Jump on the platform with the strange serpent symbol and it will lower into a Boss fight. Defeat the boss by throwing knives at the crystals on the ceiling in order to break each section of the dragon platform you are riding on. After the Boss has been defeated you will unlock Antarctica for all characters, Jim Hawkins, and the alternate ending.