The underworld of myth is where heroes go down to find what was lost. Dreaming of it is read as a descent into the unconscious, facing buried grief or fear in order to come back changed.
Does the traditional reading fit your dream, or did it mean something else to you?
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.