Poseidonis was the last isle of foundering Atlantis, and remained above water for some centuries after the drowning of the rest of the lost continent.
A large island in the Atlantic ocean, Poseidonis was a country of fertile inland valleys and semitropic jungles surrounding a central range of snow-capped mountains. The capital city was Susran; other important cities were the seaports of Lephara, Umb, and Pneor.
Sorcery was a central study of the scholars of Poseidonis. Among the greatest of the isle's sorcerers were Malygris, who for many years had effective rule over all Poseidonis, and his pupil Avyctes. Poseidonian scholars also pursued advanced studies in a range of sciences, including geology, chemistry, biology, astronomy, and atomic physics. Just before the isle finally sank, two Poseidonian scholars, the brothers Hotar and Evidon, successfully built a spacecraft and traveled in it to Venus, but did not survive long after landing.
The following Clark Ashton Smith stories include references to Poseidonis:
- "The Last Incantation" (1930)
- "A Voyage to Sfanomoe (1931)
- "The Double Shadow" (1932)
- "The Death of Malygris" (1934)