In Helaman 11:10 Nephi tells the Lord, "behold this people repenteth; and they have swept away the band of Gadianton from amongst them insomuch that they have become extinct, and they have concealed their secret plans in the earth."
Question: Nephi said that [the Gadianton band] had become extinct and they had concealed their secret plans in the earth. You [Hugh Nibley] said that meant they went underground?
Answer: I mean they literally went underground if they buried their secrets in the earth. They will dig them up later on--don't worry. These things are hid and dug up. You find them. They put them in caves and their sacred places. If you go near those places, you are in real danger because those places are carefully guarded. They have all sorts of things in them. I had quite an adventure in one where you would never expect there was anything at all. They had the whole thing down underground. [Hugh W. Nibley, Teachings of the Book of Mormon, Semester 3, p. 267]