Although I do not like to encourage such speculation about giant worms, I will go against my better judgement and post this.
I found this in Mars Express data orbit 24. I have seen these before on the Moon and they look to me like lug-worm holes that you find on the beach on Earth. It is not like a normal crater as it appears to go down evenly all around until it gets to a point or hole at the bottom.
On Earth, the lug-worm tunnels have one end has the worm cast, and the other end of the tunnel has the entry hole. This might be the entry hole.
Some of these photographs are large (50M + ) because they are complete strips. There is lots and lots of interesting stuff in them. Recommended viewing.