Symplegades

Clashing Rocks, also known as the Cyanean Rocks, were, according to Greek mythology, a pair of rocks at the Bosphorus that clashed together randomly. They were defeated by Jason and the Argonauts, helped by the advice of Phineus. 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 albeit Athena gave then a helping hand Then Athena with her left hand thrust back one mighty rock and with her right pushed the ship through. After that, the Symplegades stopped moving permanently.

The Symplegades are sometimes identified with (or confused with) the Planktai or Wandering Rocks, which are mentioned in the Homer's Odyssey and Apollonios Rhodios' Argonautica. In Apollonius's telling, the Symplegades were encountered on the way to the Golden Fleece and the Planktai were encountered on the return voyage.

The similarities and differences between the Wandering Rocks and the Symplegades has been much debated by scholars, as have potential locations for them. But the satellite image below gives pretty compelling evidence of where these rocks were historically located

Pliny the Elder says the Symplegades are the Fanari Islands, and explains that "the story is due to the fact that they are separated by so small a gap that by persons entering the Black Sea directly facing them they were seen as two, and then when the line of sight became slightly oblique they gave the appearance of coming together." (Rackham, 1942.)

It is referenced in the following articles

Jason and the Argonauts
The Odyssey and the Argonauts
Thynias