Due to the unique geological structure of Edinburgh, the city had to build two bridges to expand: The South Bridge and The North Bridge. Under the South Bridge, people established businesses, workshops, and tenements inside the arches supporting the bridge from below. However, because of the terrible sanitary terms, the city council decided to evacuate the place. So all the legitimate traders went out and were replaced by criminals and shady businesses that took their place: gambling, prostitution, bodies trade, etc.
We took a tour of one of these places with a company called Mercat Tours. Their guide was excellent, and the site is fascinating. I'm sure some other companies have a similar plan, and they are all located on the Royal Mile. The meeting point for the tour we took is at the Mercat Cross.
It was undoubtedly one of the highlights of our trip.