Is it related to any of those from the Wario Land games? Did Wario himself ever explore it in the past?
Because based on her story scenes in that game, it seems like Mona had become an Indiana Jones style explorer at one point, and it makes me wonder exactly what the place she was exploring was.
I have wondered this myself when I first played the game and I think it was explored by Wario before, because why would a treasure hunter like him leave such an easily accessible temple unspoiled?
Maybe that temple is related to the Temple of Form.