Which feature of Greece's geography explains the ancient Greeks' need for trade?

Mainland Greece is a mountainous land that is almost completely surrounded by Mediterranean Sea. There are almost 1400 Islands in Greece, it summers are hot and dry while the winters are mild and long. As the farming didn't produce huge surpluses and the terrain was difficult for travel, the Greeks had to depend on the sea. The people living near Ionian, Aegean and Mediterranean seas became sailors, merchants and Sailors. They competed with the traders from others Mediterranean cultures.