The noun form of plunge

plunge, noun, /plʌndʒ/

1 take the plunge

to decide to do something important or risky, especially after thinking about it for a long time. 
to do something after thinking about it especially for a long time.

We took the plunge and set up our own business.
We've been thinking of buying a new car, and yesterday we finally took the plunge. [=we bought a new car]
They're talking about marriage, but they don't know if they're ready to take the plunge.

2 

a sudden large decrease in the price, value etc of something. 
a sudden quick fall in amount, value, etc.

plunge in a dramatic plunge in house prices.
Market analysts predicted a price plunge.
The store experienced a sharp plunge in sales.

3 

a sudden movement down or forwards. 
a sudden fall or jump usually from a high place.

The plane began a headlong plunge towards the Earth.
Amazingly, the cat survived its plunge from the building's roof.

4

when someone suddenly becomes involved in something new. 
the act of suddenly beginning to be in a particular condition or situation.

his sudden plunge into marriage.
his plunge into severe depression.
A series of bad management decisions are responsible for the company's plunge into debt.

5 

a jump or dive into water, or a quick swim.

a quick plunge in the lake.