It is not the only case, there are news throughout history of islands vanished by natural causes and catastrophes, also ghost islands placed on maps by jealous cartographers to unmask plagiarism and unfair competition. However, the case we are dealing with here has certain connotations that make it uniquely attractive.
Claudius Ptolemy was a Greek-Egyptian geographer who worked in the Library of Alexandria during the 2nd century AD. Around 150 A.D. he published what would become one of his most important works, Geographia, in which he described the world as it was in his time.
The manuscript of the work was accompanied by maps, none of which survived. All that exist today are the work of medieval copyists (Byzantine monks under the direction of Maximus Planudes around 1295), but in each of the manuscripts it is indicated that Agatodemus of Alexandria delineated the entire inhabited world according to Claudius Ptolemy’s eight books on Geography.
According to some authors, this Agatodemus may have been a contemporary of Ptolemy, but little or nothing else is known about him. What seems clear is that he created the maps or copied them later. These maps must have played an important role in the expansion of the Roman Empire towards the East, but the work gradually fell into oblivion in the Western world. However, Muslim cartographers already used replicas in the 9th century.
After the rediscovery of Geographia by Planudes in 1295 and its translation from Greek to Latin in Florence in 1406, they would be the basis of Renaissance cartography.
But let’s go with the island itself. In Ptolemy’s 9th Map of Europe (Nona Europeae Tabula, published in 1467 and preserved in the National Library of Poland), as well as in many subsequent maps, there is an island located in the Black Sea, at the height of the present city of Rezovo and the border between Bulgaria and Turkey.
It is not a small island, it could be compared to the current Greek island of Thasos (about 398 square kilometers). The problem is that nowadays there is no island at that location.
On the maps it bears the name Cyanida (Blue) and on some of them it takes the form of two islands separated by a small channel. The fact is that after the 15th century it disappears completely from maps and never reappears again.
But let’s go back to the ancient sources. Herodotus comments on Darius’ expedition against Scythians:
After Darius left Susa he reached the Bosphorus of Chalcedon, the place where the bridge had been built; entering a ship, he went to the Cyanea islands, as they are called; of which the Greeks say they were in ancient times vague and wandering. Seated later in the temple of Jupiter Urio, he was contemplating the Pontus, because it is something that deserves to be seen, there being no sea so admirable.Herodotus, History IV-LXXXV
After the Cyanea, the navy was on its way to the Danube, and having sailed the river for two days from the sea, there they built a bridge over the cervices of the Istro, that is, in the place from where it begins to divide into several mouths.Herodotus, History IV-LXXXIX
From Herodotus’s description it can be deduced that the island (or islands) was the first to be found on the way of the ships that entered the Black Sea from the Bosphorus, just as the ptolemaic maps show.
In the early 1980s, a Bulgarian professor of geomorphology and cartography at Sofia University named Dinyo Kanev, who died in 1997, researched the underwater bottom in front of the cities of Rezovo (Bulgaria) and Igneada (Turkey). In his study, published in 1982, he asserts that there is geomorphic evidence of a submerged or sunken island about 4 or 5 kilometres from the coast: there is a large sandbank with submerged rocks, probably the remains of a destroyed island.
Since the island had disappeared from the maps at the end of the 15th century, he concluded that it must have sunk some 500 years ago. In addition, he suggested that the sinking could have been caused by a combination of tectonic movements and erosion. With the help of earthquakes and landslides, the constant breaking waves gradually destroyed the island of Cyanida, according to Kanev.
Nor does it lack identification with the mythical Symplegades, sometimes called Cyanea Rocks in mythological texts. They are the famous floating rocks that collided randomly against each other, and that the Argonauts manage to cross successfully on their way to the Colchis. In the Odyssey they are cited by the sorceress Circe as a warning to Odysseus, placing them close to the monsters Scylla and Charybdis:
No boat of men, in arriving there, was able to escape unharmed; For the waves of the sea and the storms, laden with pernicious fire, carry together the boards of the ship and the bodies of men. The only thing that managed to bend those rocks was a ship furrowing the ponto, Argo, so celebrated by all, on returning from the country of Eetes; and also this one would have been crashed by the waves against the great rocks, if Hera had not made it pass by them because of her affection for Jason.Odyssey, XII-55
Apollonius of Rhodes also mentions them in his Argonautics, an epic poem written in the 3rd century BC:
As soon as you leave me, the first thing you will see are the two Cyanea Rocks, there in the place where the waters of the sea are narrowed. I assure you that no one can escape safe and sound unless he tries to cross them, for they are not rooted in deep roots, but rather they continually go to meet each other colliding with each other, and an enormous mass of water piles up over them bubbling, and the firm rock of the coast roars around with penetrating noise.Apollonius of Rhodes, Argonautics II-319
In any case, the version of the natural collapse would be supported by the evidence of remains of different ancient cities submerged on the coasts of the Black Sea: Byzone, present-day Kavarna; Acra, near present-day Chernomorets; or Messembria, present-day Nesebar, among others.