Found it - it's this mound (sort of upper right'ish). Gets the Thumper thing - then the worm comes - then you need the pieces inside. Then thumper your way out.
Hold on - I just saw the screenshot here - am I missing yet another piece?
Looks like I am missing a circular piece?
https://itch.io/post/2665606
4 years ago(+1)
If you failed in the worm once, did you have a problem with your mouse cursor vanishing the second time?
I didn't have an issue going back in. However, someone else reported a camera issue.
https://itch.io/t/1219734/v4-re-entering-sandworm-breaks-camera