This is essentially a cave (or something similar to a cave) built into or connected to the pool, typically with a sitting area. It is common to build waterfalls or weeping walls on the outside of the grotto, and/or a rain wall at the entrance. Youngsters love it when a slide is built in to (on top of or to the side of) the grotto. If you build it large enough, you can actually put your spa inside the grotto, to offer extra privacy, and a more romantic mood.
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