what was the most important discovery in this story Why do you think so?

The things they witness below the surface of the Earth are things that no one else (except Saknussemm, putatively) has ever seen, and Verne depicts the characters' stunned reactions in a memorable and believable way. I think the discovery at the ending was pretty cool. They find a seemingly bottomless pit beyond the impeding rock and are swept into it as the rushes down the huge open gap.