The Tenderloin district in San Fran (Named after several theories: New York district, corrupt police saying, the loins of the prostitutes or the shape of the area). Here's all I saw in one go. Hidden gem is "The Magazine" shop on Larkin which Robert Crumb frequented. The owners have been there 45 years and give a free Crumb poster if asked. Most shocking thing I saw was an amputee with huge knife in his belt. Completely illegal and instant arrest in London.