The PURPLE CAVE is not far from the BLUE CAVE (see Post #3).
It is also an alien laboratory -- but a different species. As the characters in the screenplay investigate the cave, they are startled to realize that each step they take produces a weird musical note. One of the characters cracks the musical code, which reveals the aliens' alphabet...
(1999/Alesis) (Track 12 on CD)
A Purple Cave (1:18)