The Grotto is a place in Mako Island Of Secrets. It appears to be a cavern with several lavish furnishings and it is quite possible that the hidden bookcase entrance in Rita Santos's house enters into the Grotto. There is also a tunnel that leads to the ocean down the hallway from the Grotto. It resembles the Moon Pool.
There are altogether 2 entries to the Grotto, a bookcase entrance and an underwater sea cave entrance. To get in through the bookcase you pull the book with mermaids on the spiral and it will pull a bookcase apart creating two bookshelves and you can walk in and it will close after everyone in the room has entered. The underwater entrance can be accessed by Rita and The Mermaids through the sea. Only rita, the mermaids, Zac and Cam know.