Poseidon (Greek: Ποσειδῶν) was the god of the sea, and, as "Earth-Shaker," of the earthquakes in Greek mythology. The name of the sea-god Nethuns in Etruscan was adopted in Latin for Neptune in Roman mythology: both were sea gods analogous to Poseidon. Linear B tablets show that Poseidon was venerated at Pylos and Thebes in pre-Olympian Bronze Age Greece, but he was integrated into the Olympian gods as the brother of Zeus and Hades. Poseidon has many children. There is a Homeric hymn to Poseidon, who was the protector of many Hellenic cities, although he lost the contest for Athens to Athena.
Poseidon was a major civic god of several cities: in Athens, he was second only to Athena in importance, while in Corinth and many cities of Magna Graecia he was the chief god of the polis.
In his benign aspect, Poseidon was seen as creating new islands and offering calm seas. When offended or ignored, he supposedly struck the ground with his trident and caused chaotic springs, earthquakes, drownings and shipwrecks. Sailors prayed to Poseidon for a safe voyage, sometimes drowning horses as a sacrifice; in this way, according to a fragmentary papyrus, Alexander the Great paused at the Syrian seashore before the climactic battle of Issus, and resorted to prayers, "invoking Poseidon the sea-god, for whom he ordered a four-horse chariot to be cast into the waves."
According to Pausanias, Poseidon was one of the caretakers of the oracle at Delphi before Olympian Apollo took it over. Apollo and Poseidon worked closely in many realms: in colonization, for example, Delphic Apollo provided the authorization to go out and settle, while Poseidon watched over the colonists on their way, and provided the lustral water for the foundation-sacrifice. Xenophon's Anabasis describes a group of Spartan soldiers in 400–399 BCE singing to Poseidon a paean—a kind of hymn normally sung for Apollo.
Like Dionysus, who inflamed the maenads, Poseidon also caused certain forms of mental disturbance. A Hippocratic text of ca 400 BCE, On the Sacred Disease says that he was blamed for certain types of epilepsy.