re: From the image that Pete B. posted below....
... looks like there are
'two caves' (excellent image!). If they can't maneuver Spirit tiwards the larger 'cave', the smaller one could be an easier reach. A longer trek around the 'hills', but worth investigating (imho). Interesting terrain surrounding those caves. From looking at this image, instead of something going in to create the caves/voids as I first thought, it looks like 'stuff' (rocks, gas...???) was projected outwards.