In Freudian analysis, the symbol of the underground showing up in your dream was a symbol rooted in sexuality and needs repressed deep in the unconscious. Although most interpretations today would find his theories controversial, the great dream doctor was not too far off in his suggestion. Dreaming of the underground can often imply a sexual connotation, with many metaphors and innuendos that can be created from the world alone.
There is nothing wrong with seeing this show up in your dream, particularly if that part of your life needs a little bit of a shake up. It’s a human experience and need that we all have, and sometimes life just needs a little tweak to feel the spark of renewal again. So this could very well be the literal interpretation of tunnels or the underground appearing in your dream.
A more metaphorical interpretation could be the application of you delving deeper into your subconscious, or becoming more intimate with your subconscious in a way that only dreams can allow. What are the feelings associated with your experience of the underground? Are you fearful or excited? Is the underground dream suffocating you, or are you seeing your way through it clearly via tunnels and light?