The symptoms of Parkinson's Disease appear when the dopamine levels in the brain decline. Dopamine is produced in the brain in the Substantia Nigra, which is located deep in the brain, and is one of the parts of a bigger area called the "basal ganglia".
In Parkinson's, the cells of the Substantia Nigra lose power and their dopamine production slowly declines.
The images below show where the Substantia Nigra is located. The images are taken from different angles and collectively show just how deep inside the brain this area is.
Look for the light blue blob.
Looking at the brain from the front:
Looking at the brain from the side:
Looking at the brain from the top:
Looking at the brain from the back:
These images came from the app 3D Brain.