The Gigantomachy was probably the most important battle that happened in Greek mythology. It was a fight between the Giants or Gigantes, sons of Gaea and Uranus, and the Olympian gods who were trying to overthrow the old religion and establish themselves as the new rulers of the cosmos.
According to the most detailed source for this battle, what started the war was the Giant Alcyoneus stealing the cattle of god Helios. A prophecy had it that the Giants would only be defeated if a mortal was to help the gods. To protect her children, Gaea tried to find a plant that would shield the Giants from any harm; however, Zeus stopped Eos (Dawn), Selene (Moon) and Helios (Sun) from shining, and took every single plant for himself. When the battle began, Heracles fought Alcyoneus; however, the Giant would not die as long as he stepped on the soil of his homeland. With Athena's advice, Heracles dragged Alcyoneus away from his homeland, and killed him. The other Giants had similar fates; Dionysus killed Eurytus; Athena buried Enceladus under Sicily; Hermes killed Hippolytus; and so on. Many of the Giants were buried under islands. In fact, it was believed that the earthquakes and the volcanic eruptions were caused when the Giants moved in their tombs.