After the fall of the Titans, the three brothers Poseidon, Zeus and Hades drew lots to divide the powers of the world fairly.
Zeus' prize was heaven and he became the supreme ruler of the Greek gods. Hades' prize, on the other hand, was a rather unpleasant one, for he was chosen by fate to maintain the underworld.
Poseidon's prize was the seas and so Poseidon became the Lord of the Seas and second in power among the Greek deities.
Only the earth and Mount Olympus remained communal.