Xiriux, I thought I had been the first to spot that opening in the ground - but I see now that you beat me to it, and that your analysis is similar to mine.  :)  Here are a couple of my images of it:
Hi Chandre, yes I can imagine that the object leads underneath the surface. I have examined the object you marked and I have found using 3D anaglyph method that it is a strange shaped object with brightung top, but its form is not a "dome". It is put on a big light rectangular stone. I recommend you to use the anaglyph way. The recongnition will be better and the error rate is far less than in 2D. If you examine this navcam image pair in 3D anaglyp you can find strange, ruin-like structures in the valley. Check it. Here is a link: http://stereo.jpn.org/eng/stphmkr/
You can download from here a very good japanese free program to create and analyze anaglyph and other stereo images.
Hi Xiriux, nice find ! Do you think it could be an underground entrance ramp ? The object that I was wondering about is below, it seems to have a small dome similar to the one you posted.
My opinion that it's a cloud-on-surface (of dust? of water vapour?). If you analyze both right and left image, you can see that this cloud moved in the interval between taking right and left photo. About tampering: the hill and the far scene are moderately blurred although the depht in field of the navcam lenses is very large: from 0.5 m- to infinity. I.e. they should be sharp , not blurred. Chandre, I say again that your sight is very good: yes theare are a lot of interesting structure on the slope and in the valley, but because of blurring I am unable to recognize them There is a strange surface form in the middle of your attached image: a trapezoid shape structure (check it) with relatively large size. It's very interesting that also there is a similar structure on the west plane at the foot of Columbia Hills : look below.
The object pointed by blue arrow is an obviously unnatural trapezoid structure: its height is about 2 m, the base of tha trapez is about 5 m, and the width of the object is about 5-6 m considering the perspectivical foreshortening. Stange circmstance is that the object is cavernous: the black patch is not simply a shadow but a cavernous part in shadow.Look the other side of the structure: it is lilluminated by the Sun. The yellow arrow marked object is an unrecognizable thing on the top of a heap, but it overtowers over the surface by more meters.
Really nice find ARP2 ! There are lots of interesting objects in this image. It looks like it has the smudge tampering that Skipper mentions, but not well applied. There are some interesting objects/shapes in the dark area under your 'round' area that would be nice to see a bit more clearly. There is also a strange 'rock' formation in the middle of the image about two thirds of the way up with unusual angles and shadows that may be an anomaly ? It looks like it has a similar 'glass dome' to the one that Xiriux has shown.
I have another hill anomaly for scrutiny... this is from Spirit Nav Cam 592.� To the left of the image, it appears there is either mist, clouds or steam?� Also to the right of that, there appears to be some pipe sticking out of hill?� Last, to the right of the "pipe", appears a strange pixel distortion, or something looking round.� If you look at the pixels closely there, you will see a curve to the pixel... very odd. ARP2