On the Greek side was division; the Spartans imagined that their duty was to save the Peloponnese only; they were eager to build a wall across the isthmus of Corinth, leaving the rest of Greece to its fate. But Athens had produced another genius named Themistocles. Shortly before the invasion the silver mines at Laureium in Attica had yielded a surplus; he persuaded the city to use it for building a fleet of two hundred sail to be directed against Aegina. When the Athenians got an oracle from Delphi which stated that they would lose their land but be saved by their wooden walls, he interpreted the oracle as referring to the fleet. Under his management the city built more ships. The Council of Greece held at the Isthmus of Corinth decided that an army should defend Thermopylae while the fleet supported it close by at Artemisium. The Persian fleet had been badly battered in a storm as it sailed along the coast of Magnesia, nearly four hundred sail foundering; the remainder reached safe anchorage in the Malian gulf, further progress being impossible till the Greek navy was beaten or retired. At Thermopylae the advance-guard was composed of Spartans led by Leonidas who determined to defend the narrow pass. A Persian spy brought the news to Xerxes that this small body of warriors were combing their hair. The King sent for Demaratus, the ex-Spartan monarch, who assured him that this was proof that the Spartans intended to fight to the death. After a delay of four days the fight began. The Spartans routed all their opponents including the famous Immortals, the Persian bodyguard. At length a traitor Ephialtes told Xerxes of a path across the mountains by which Leonidas could be taken in the rear. Learning from deserters and fugitives that he had been betrayed, Leonidas dismissed the main body, himself advancing into the open. After winning immortal glory he and his men were destroyed and the way to Greece lay open to the invader. In three naval engagements off Artemisium the Greek fleet showed its superiority; a detachment of two hundred sail had been sent round the island of Euboea to block up the exit of the channel through which the Greek navy had to retreat, but a storm totally destroyed this force. When the army retreated from Thermopylae the Greek ships were obliged to retire to the Isthmus; in spite of much opposition the Athenians compelled Eurybiades the Spartan admiral to take up his station at Salamis, whither the Persian navy followed. Their army had advanced through Boeotia, attacking Delphi on the way. The story was told how Apollo himself defended his shrine, hurling down rocks on the invaders and sending supernatural figures to discomfit them. Entering Attica the barbarian host captured a deserted Athens, Xerxes sending the glad news to his subjects in the Persian capital.