A sea change

Meaning
A radical, and apparently mystical, change.

Origin
From Shakespeare's The Tempest, 1610:

ARIEL [sings]:
Full fathom five thy father lies;
Of his bones are coral made;
Those are pearls that were his eyes:
Nothing of him that doth fade
But doth suffer a sea-change
Into something rich and strange.
Sea-nymphs hourly ring his knell

Source: http://www.phrases.org.uk/meanings/312800.html