There are seven groups of Aegean islands: the Thracian Sea group, the East Aegean group, the Northern Sporades, the Cyclades, the Saronic Islands (or Argo-Saronic Islands), the Dodecanese and Crete. The word archipelago used to mean these islands. Many of the Aegean islands, or chains of islands, are part of the mountain ranges on the mainland. One chain goes across the sea to Chios, another one goes across Euboea to Samos, and a third one goes across the Peloponnese and Crete to Rhodes. This one divides (splits, cuts) the Aegean from the Mediterranean. Many of the islands have safe harbours and bays, but sailing in the sea is difficult. Many of the islands are volcanos, and marble and iron are mined on other islands. The bigger islands have some green valleys and plains. Two big islands on the Aegean Sea are part of Turkey: Bozcaada () and Gökçeada ().