Note this image from today's APOD
Spirit's exploring grounds around Home Plate - note the
dark region at the base of Husband Hill called El Dorado:
http://antwrp.gsfc.nasa.gov/apod/ap071212.html
I can recall when folks wondered if it was a cave due
to being so dark. That's what I think this "door" image
is as well.
Of course we have to be careful going to the other
extreme: Suppose an artificial non-terrestrial object
is found in the Sol system some day. Will it be ignored
and dismissed out of fear of ridicule et al?
