Guanajuato is a mountainous city with very narrow roads. Therefore, abandoned silver mine tunnels near the city were transformed into streets for vehicles, and pedestrian walkways are also provided on both sides of the tunnels. If you take a taxi to Guanajuato, it will almost always use these underground passages. Sometimes Google Maps will also guide you to a few walking routes that utilize these underground streets. It's not really worth making a special trip there; there's not much scenery to see, but you can experience it for the time being.