A vast and dangerous realm that existed on the planet Eternia which was actually the name of a large system of caverns as well as pathways that were located underground. It was known to be the home of both the Speleans and Caligars; two species who were antagonistic to one another. Whiplash who was Caligar was known to had grown up in this realm. Subternia was also home to the Temple of Serpos that was a religious site of the Snakemen and contained the Amulet of Serpos which was hidden away for centuries. The Caligars were known to had made an opening to Subternia near a volcano with this site being the location of a piece of the Triad of Discidium.
In later years, a dispute emerged between the Speleans and Calligars which prompted the intervention of King Randor who sought to bring peace between the two races. At the same time, Skeletor used Subternia as a means of attacking Castle Grayskull from below but was defeated by He-Man. Portions of Subternia faced considerable damage when the Spell of Separation was accidently envoked by Twobad. Later, King Hiss hid in Subternia after his initial defeat and used its caverns to enter into the Sea of Rakash. Afterwards, the King of the Snake Men used captured Caligars to excavate the Temple of Serpos in order to retrieve the ancient Amulet of the Snake God Serpos as part of his plans.
He-Man and the Masters of the Universe (2002 Series)