You have explored everything you could so far, but there must be something more. You just need a stepping stone to the next challenge.
In the imager message that Atrus left for Catherine, Atrus talked about his sons and also said, "...and don't forget the tower rotation." Where is that tower he's talking about?
I've already figure out the tower rotation, but...
Go back to Myst Island -- the beginning of the Myst hint area