cold
Bathers would go straight from the hot room to take a cold plunge.

downward, headlong (both figurative)
the economy's downward plunge
his headlong plunge into shame and hypocrisy

survive
How did he survive his icy plunge?

take (usually figurative)
She prepared to take the plunge into the cold waters of Lake Ontario.
He finally took the plunge and gave in his notice.

plunge into
a plunge into the icy water

plunge to
a plunge to the ground

ahead (AmE,often figurative), back, downwards/downward, forward, head first, headlong
City officials are plunging ahead with plans for a new convention center.
The car plunged headlong into the river.

in
The pool was declared open and eager swimmers plunged in.

down
The bus came off the road and plunged down an embankment.

from
He plunged from a tenth floor window.

into (often figurative)
She plunged straight into her story.

off
The car had plunged off the road.

plunge to your death
A climber plunged 300 feet to his death.

Plunge is used with these nouns as the subject:
horse, price, profit, rate, stock

Plunge is used with these nouns as the object:
knife, needle, per cent, point