The secret entrance to the Martian Underground...
...it was only a matter of time before it was imaged. Let me compare it with a CO2 map of the region and we'll see if thats one of the exit points for the thriving city below...if it is, we can visit some of our dismantled, disabled spacecraft in the 'museum of humanity'.
;-)
Oh man, conspiracy-folks are going to have a field day with this one almost as big as the 'Face', perhaps bigger.
Which reminds me, if we can't see sidewall features this hole is even bigger than it looks and expands underneath dramatically after the rim of the opening. Can you see the space port to the left? What about the two ancient pyramids to the northwest? ;-)