Stillwater Grotto is a secret cave with an I-Point in it.
It can be found near Weathered Monument.
When you go inside, it is a dead end, but somewhere at the top, there is a secret exit.
The description in the game is simply: "Watch Out!" in the day. It is accessible by
a tunnel with a large sand mound at the entreance.
In Wii Fit and Wii Fit Plus there is a bridge that is in a spiral allowing you to go into the secret exit.