Day time gods and night time gods are frequently deities of an upper world or celestial world opposed to the earth and a netherworld (gods of the underworld are sometimes called chthonic deities). Within Greek mythology, Uranus was the primordial sky god, who was ultimately succeeded by Zeus, who ruled the celestial realm atop Mount Olympus. In contrast to the celestial Olympians was the chthonic deity Hades, who ruled the underworld, and Poseidon, who ruled the sea.