Let me look at both the underground and violence. First of all, “underground” is not a place. It's impossible for prostitution to go underground. In order for buyers to be in contact with those they want to purchase in prostitution, there needs to be a communication.
That can be through advertisements; it can be online; it can be in the newspapers, as was shown by the Adult Entertainment Association of Canada. There are different methods of making contact. As much as the buyers can get in contact with the women or the men who are in prostitution, so can the police, and the Swedish police have spent the past many years doing investigations online on websites that advertise the sale of women and their purchase by men in Sweden.