I was looking at some of the pictures sent back by the Spirit Mars rover.
I've see a few videos and picture showing strange things in the nasa images and I've even seen videos about this picture on places like youtube.com and ufotv.com but I don't think anyone has noticed this before.
09-MG-06-sol178-A190R1.jpg
From: http://marsrovers.nasa.gov/gallery/press/spirit/20040716a.html
This is a section of the picture above of the hill or mountain in the distance. The blue arrow is pointing at the circle.