Here's a couple of interesting objects on Mars' surface, but spotted by the Mars Global Surveyor.
The first in in the Ceti Mensa region and appears to be a disk-shaped object, that has skidded to a halt, embedding itself in the sand, gouging a trench behind it, into which sand has fallen and dunes formed. The disk is estimated to be 12 to 15 metres diameter.
The entire video is worth watching for a detailed analysis:
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The next is of three perfectly aligned objects, resembling conical towers, in the Terra Meridiani region.
They are estimated as being 240 metres apart and several hundred metres tall.
The video is in Spanish, with close-ups and analysis starting around the 3 minute mark. He also describes other apparently anomalous objects in the vicinity:
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