In Greek mythology, the Symplegades were a pair of rocks at the Hellespont that clashed together randomly. They were defeated by Jason and the Argonauts, who would have been lost and killed by the rocks except for Phineas' 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 Romans called them cyaneae insulae.

Apollonius Rhodius. Argonautica II, 317-340, 549-610

First of all, after leaving me, ye will see the
twin Cyanean rocks where the two seas meet. No one, I ween, has
won his escape between them. For they are not firmly fixed with
roots beneath, but constantly clash against one another to one
point, and above a huge mass of salt water rises in a crest,
boiling up, and loudly dashes upon the hard beach. Wherefore now
obey my counsel, if indeed with prudent mind and reverencing the
blessed gods ye pursue your way; and perish not foolishly by a
self-sought death, or rush on following the guidance of youth.
First entrust the attempt to a dove when ye have sent her forth
from the ship. And if she escapes safe with her wings between
the rocks to the open sea, then no more do ye refrain from the
path, but grip your oars well in your hands and cleave the sea's
narrow strait, for the light of safety will be not so much in
prayer as in strength of hands. Wherefore let all else go and
labour boldly with might and main, but ere then implore the gods
as ye will, I forbid you not. But if she flies onward and
perishes midway, then do ye turn back; for it is better to yield
to the immortals. For ye could not escape an evil doom from the
rocks, not even if Argo were of iron. Argonautica II, 317-340,