Please answer a question about the following article about Greece:

The Pindus range continues through the central Peloponnese, crosses the islands of Kythera and Antikythera and finds its way into southwestern Aegean, in the island of Crete where it eventually ends. The islands of the Aegean are peaks of underwater mountains that once constituted an extension of the mainland. Pindus is characterized by its high, steep peaks, often dissected by numerous canyons and a variety of other karstic landscapes. The spectacular Vikos Gorge, part of the Vikos-Aoos National Park in the Pindus range, is listed by the Guinness book of World Records as the deepest gorge in the world. Another notable formation are the Meteora rock pillars, atop which have been built medieval Greek Orthodox monasteries.

Islands in the Aegean Sea are formed by what?
underwater mountains