Here is the full size image:
GigapanGigapan - http://www.gigapan.com/gigapans/229311 GIGAmacro - https://viewer.gigamacro.com/view/GIvcoFX3fWdxwa3V?x1=33436.50&y1=-13865.50&res1=34.83&rot1=0.00 Flickr Download - https://www.flickr.com/photos/nev-t/52063976257/sizes/6k/ Raw Images -...
http://www.gigapan.com/gigapans/229311
Zoom in towards the center of the picture and you see this:
NASA says it's a natural crevice in the rock, and that it's also very tiny because the picture is zoomed in so much. The opening in the rock is 30x45cm.
But maybe it's really a door for tiny martians.