Please explain this ETS Official Guide question

Can someone explain Blank 2?

1- rumination on
2- thoroughly commonplace
3- most prosaic
Are these right answers?

The correct answers are rumination on, utterly mysterious and most prosaic. I am confused about Blank 2 and don’t know why the official answer is mysterious

I think there are 2 clues which suggest the answer must be utterly mysterious: 1- phrase “to deepen our understanding” 2- In last sentence the word “finding” - we usually try to deeply understand and find something about a mysterious matter.
I hope you get my point.

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