Going down into a mine is read as digging into the unconscious for what is buried and valuable. Dark, narrow tunnels can also stand for heavy work and feeling far from the light.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.