Wikipedia
The Symplegades (; , Symplēgádes) or Clashing Rocks, also known as the Cyanean Rocks, were, according to Greek mythology, a pair of rocks at the Bosphorus that clashed together when somebody went through. They were defeated by Jason and the Argonauts, who would have been lost and killed by the rocks except for Phineus' advice. Jason let a dove fly between the rocks; it lost only its tail feathers. The Argonauts rowed mightily to get through and lost only part of the stern ornament. After that, the Symplegades stopped moving permanently.
The European rock is usually identified with an islet, about wide and long, which stands about off the shore of a village called Rumeli Feneri ('Lighthouse of Rumeli), and is connected to it by a modern concrete jetty. At its highest point, there is an ancient altar known as the Pillar of Pompey, though it has nothing to do with Pompey. Dionysius of Byzantium mentions a Roman shrine to Apollo on one of the Cyanean Rocks, and the 16th-century French traveller Petrus Gyllius thought the altar was a remnant of that shrine.
The Asian rock is probably a reef off the Yum Burnu (north of Anadolu Feneri 'Lighthouse of Anatolia'), described by Gyllius:
The reef is divided into four rocks above water which, however, are joined below; it is separated from the continent by a narrow channel filled with many stones, by which as by a staircase one can cross the channel with dry feet when the sea is calm; but when the sea is rough, waves surround the four rocks into which I said the reef is divided. Three of these are low and more or less submerged, but the middle one is higher than the European rock, sloping up to an acute point and roundish right up to its summit; it is splashed by the waves but not submerged and is everywhere precipitous and straight."