Poseidon was the Greek god of the sea, earthquakes, and horses. He was a powerful yet unpredictable god who was feared by sailors. As the god of the sea, he controlled oceans and could cause earthquakes by stamping his foot or striking the earth with his trident. The Greeks saw similarities between the unpredictability of earthquakes, oceans, and horses. Poseidon competed with Athena to be the patron deity of Athens but ultimately lost, though the Athenians still honored him.