sea change
noun phrase

- a profound or notable transformation

(Oxford Dictionary)

---
ORIGIN

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.

This lyric is recited by Ariel in Shakespeare's "The Tempest" and is thought to be the origin of the expression "sea change." The song refers to Ferdinand's father who died by drowning.

With "sea change," Shakespeare is talking about how the sea transforms a dead body, not really a profound change. But perhaps because the oceans of the earth are so vast and powerful, this expression came to be associated with change of a large magnitude.

In the first line, "full fathom five" is a measurement of water depth. One fathom is around six feet (1.8 metres), so that in the lyric Ferdinand's father is lying at a depth of around 30 feet or 9 metres.

Before measurement instruments were invented, a fathom was determined by measuring the distance - from fingertip to fingertip - of a man's outstretched arms (between five and six feet).

The term fathom derived from the Middle English "fathme," meaning embracing or outstretched arms.

---
Practice OWAD in a conversation:

"The global economy is undergoing a SEA CHANGE as a result of the pandemic."