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View into the Grotta del Ninfeo that is an artificial cavity in the rock of the Temenite Hill at the back top of the Theater. The water flowing in the back is coming out of a crack in the rock, that in turn is fed by one or two aqueducts. Behind the horizontal barrier that is carved into the living rock is a basin from which water could be drawn.
The grotto was dedicated to the Nymphs and thus is called a Nymphaeum.
In the late 1800s water flowed down from the basin and powered water mills that were located IN the theater!
For additional information see Wikipedia Here.