Hi, Nick!

You've pooh poohed the idea of caves or even cubbyholes of sorts
existing in that steep cliff, and beneath the overhang on the opposite side
of Endurance Crater. 'Opposite' to the original arrival location of the Opportunity rover.
Forgive me if I'm putting words into your mouth.

Granted! Those 3 small dark dots/pixels that I can make out in that cliff-wall in the thermal imagery of that crater could simply be 'missing info' or maybe large cracks, or they night be cubby holes of sorts [anything
from a 0.3 to one meter in diameter.
I confess, you and I may never know.
So any bets regarding any 'hypothetical' orifices/cubbyholes in the cliff walls of Endurance would be futile.
So much for Endurance.

BUT!....The backside of the "Columbia Hills" is a different story.

There appear to be much larger cliffs, perhaps more precipitous than anything in Endurance, and showing evidence of "sapping" at their base.
And then there's what appears to be 'genuine' rock overhangs on those
3-d computer-generated images.
To me that looks like strong evidence for the existence of a cave
large enough for a human to crawl into [if not walk into].
And what's more!...The Spirit rover has a chance [a chance] to carry out
a genuine close-up inspection of the site to prove me either right or wrong.
And I strongly believe there is a cave there!
I'm willing to wager a steak dinner [or foreign money-order equivalent]
that THERE IS a cave there on the backside of the "Columbia Hills"!
That's my wager, my Aussie friend!

cheers.

Jacob

"Extraordinary claims, require extraordinary evidence."
CARL SAGAN