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The HiRISE astronomical imaging project has a striking picture of a spot on Mars' surface that looks like total blackness to their Earth-based camera—it sees no light beyond the background noise level. The conventional explanation would be that it's a steep hole. But until we get some direct evidence I thinkwe shouldn't rule out the possibility that it's something much stranger, like a chunk of the black material that makes up the monoliths in 2001.
