The Grotto is a place in Mako Mermaids. It appears to be a cavern with several lavish furnishings, and the bookcase in Rita's house is used as a secret entrance into the Grotto. There is also a tunnel that leads to the ocean down the hallway from the Grotto, and it resembles the Moon Pool in many ways. Rita has collected many mermaid artifacts and treasures, including shells, lamps, coins, and other things.
This is the place where Rita teaches the girls their magic lessons.