Before the level of the sea rises for 120 metres, Cyclades was a single island and the Corinthian Gulf was a lake, as reveals the archaeologist-historian George Steinhauer in his book "Historical Geography of the Ancient World: Greece- Rome". As the writer explains, in the Pleistocene (Paleolithic era), the sea level after the melt of the ice rose for at least 120 metres. Before this happens, Corfu and Lefkada were still part of the mainland, as were Salamina and Aegina. This means that the deepest parts of the Corinthian and the Saronic Gulfs were just lakes.
However, there are evidences that the sea level also rose for about 2 metres a bit later, during the historical times. The remains of ancient ports (Piraeus, Epidaurus) or whole submerged towns (Pavlpetri in Elafonissos) show that these places were covered by the water, when the sea level rose. In a similar way, due to some local geological phenomena, ports in eastern Crete were submerged while the port of Falassarna in western Crete is located a few metres above the sea level.