Many of the islands are of volcanic formation; and a well-defined volcanic chain bounds the Cretan Sea on the north, including Milo and Kimolos, Santorin (Thera) and Therasia, and extends to Nisyros.
In the very early rock inscriptions of Thera (700-600 B.C.), written from right to left, it appears in a form resembling the ordinary Greek X; this form apparently arose from writing the Semitic symbol upside down.
To the student of antiquity the most interesting are: Delos, one of the greatest centres of ancient religious, political and commercial life, where an important series of researches has been carried out by French archaeologists; Melos, where, in addition to various buildings of the Hellenic and Roman periods, the large prehistoric stronghold of Phylakopi has been excavated by members of the British school at Athens; and Thera (see Santorin), the ancient capital of which has been explored by Baron Hiller von Gaertringen.
Thera is also of special interest to geologists owing to its remarkable volcanic phenomena.