Wiktionary
plunge pool
n. A pool that is small in diameter, but deep, usually resulting from a natural force such as a waterfall or rapids.
Wikipedia
Usage examples of "plunge pool".
At length he finished and let the dragon retire to the plunge pool.
The wyverns were still in the plunge pool, so for a moment they had the place to themselves.
The passage sloped slightly, making a gradient that pulled the cave runoff down into darkness on the far side of its new plunge pool.
A square plunge pool lit by green underwater lanterns took up half the echoing space.
They'd played squash, been massaged, had saunas, encountered the plunge pool, gone swimming, survived the expensively equipped gymnasium, tried jogging .