A growing part of the internet is devoted entirely to fraudulent advertising. According to a new report by White Ops, ad-fraud sites make up roughly 20 percent of ad-serving sites on the web, despite having no human traffic whatsoever. The sites make money entirely from traffic bots, designed to mimic human visits to generate ad revenue for the sites’ creators.

Traffic bots have long been a problem for web advertising, also known as “click fraud.” The good news is that, while it remains a billion-dollar industry, advertisers are slowly getting wise to the tricks. According to White Ops, ad-fraud sites are on track to make $6.5 billion in 2017, down from $7.2 billion the previous year.

The problem is particularly acute for desktop video ads, which draw criminals through high prices and a relative scarcity of legitimate inventory. The survey found that 22 percent of desktop video ads were viewed only by traffic bots, a figure that’s held steady from year to year.