After overthrowing the Titans, the three brothers Poseidon, Zeus and Hades drew lots in order to share justly the powers of the World. Zeus' price were the heavens and he became the supreme ruler of the Greek gods. The price of Hades, on the other side, was a rather unpleasant one, since he was chosen by fate to receive the Underworld.
Poseidon's price were the seas and this way, Poseidon became the Lord of the Seas and the second in power among the Greek deities.
Only the Earth and Mount Olympus remained communal.