The once mighty Demon King died, his rise meteoric and his fall just as abrupt. There's no disputing that beneath this nameless island, Typhon lies in eternal slumber.
When fate denied his existence and the power that made him chose not to sustain him, the mightiest demon of history was doomed to fall, sooner or later.
However, flying above the island, Zeus was also aware that although he had slain Typhon, the power of fate had only targeted the demon known as 'Typhon.' Beyond that, the remnants of Typhon's original essence, which he hadn't absorbed and which fate had judged to be enough to reverse the balance of power between him and his opponent, would not simply vanish into thin air.
They dispersed, in some way that Zeus could not quite comprehend. Beneath the seas, later known as the Thalassan Sea, a continuous stream of power seeped out, becoming an indispensable part of this world.