Plunge is a noun and a verb related to diving, falling, and sinking. A "polar bear plunge" is when people plunge into freezing cold water for charity or, inexplicably, for fun.
As a noun, plunge means “a brief swim” or “a fall.” As a verb, plunge has a variety of definitions. Plunge can mean “to devote yourself fully," as you might plunge into a job by working day and night. Similarly, if you plunge into a good book, you don't let anything distract you from your reading. Plunge can also mean “fall abruptly.” Someone’s reputation might plunge after a scandal, or the stock market might plunge when investors react to bad news.