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

take
The river takes a headlong plunge into a maelstrom of rocks and boulders. | (figurative) He finally took the plunge and gave in his notice.

~ into
a plunge into the icy water

~ to
a plunge to the ground

downwards, forward, head first, headlong
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.