[Magical Mystery Cache]
Swim out to the island you can see when at GZ, taking great care not to splash, or you may attract the sea serpent.
Once on the beach, close your eyes and turn counterclockwise 3 times. After three turns, open your eyes and you will see a Sphinx & 3 doors. Answer the riddle posed by the Sphinx, then open the left-hand door and pass through the gate.
In the Garden, 10 feet northwest of the Fountain, you'll find the cache hidden in a sagebrush, under some rocks.