Here's why:

* Tectonic Activity: Southern Europe is a region of intense tectonic activity, where the African and Eurasian tectonic plates collide. This collision creates mountains, both on land and underwater.

* Volcanic Activity: Many of the islands are formed by volcanic activity, with magma rising from the Earth's mantle and erupting through the ocean floor, creating new landmasses.

* Examples:

* Sicily and Mount Etna: Mount Etna, an active volcano, is located on the island of Sicily.

* The Aegean Islands (Greece): Many islands in the Aegean Sea are of volcanic origin.

* The Canary Islands (Spain): These islands are formed by volcanic activity originating from a hotspot in the Earth's mantle.

While there are undersea plains in the Mediterranean Sea, they do not play a significant role in the formation of the islands.