During a Book of Mormon lecture on this issue, Dr. Hugh Nibley had this exchange with a student:
“Question: Nephi said that [the Gadianton robbers] had become extinct and they had concealed their secret plans in the earth. You 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” (Teachings of the Book of Mormon, 3:267).