I'm perty sure you are seeing hall of mirror and refraction effects.
Rays are passing in and out of the sinusoidal surface creating
dark spots, the surface has shallow peaks along the ridges
from floating point errors in the functions. POV also doesn't
perfectly model the IOR of interiors, the dropped out portions
of light bent inside of a material are black in POV but reflected 
IRL.

The bottom line is that POV isn't a physics simulator, it's
a way to make perty pictures.

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