“Unique visitors” aren’t always that unique. Sometimes, they’re not even visitors — at least of the human variety. A story in The Wall Street Journal looks at what happens marketers end up buying ad space on websites that turn out to be visited only by robots.

While some scammers create stand-alone operations, others devise sprawling empires. In one case, the White Ops technology uncovered a zombie-populated lifestyle network, with hundreds of connected sites, including bodybuildingfaq.com, financestalk.com, and abctraveling.com. No one at the sites could be reached for comment.

In some scenarios, legitimate websites inadvertently set themselves up for botnet invasions when they hire companies to help boost their traffic. That can involve building audiences through methods such as paid keyword-search advertising with search engines.

White Ops discovered that more than 30% of the visitors to the education portal Education.com were robots. In the past month the site received about four million unique views, according to Quantcast.

A spokesman for Education.com said it was aware of the bot-traffic and that it had likely come from an initiative in the summer to boost its audience numbers. Education.com had bought traffic from a variety of legitimate sources, including search engines, to lure in new subscribers, as well as users “who would perform well for advertisers.”

“We shut down the program,” the spokesman said.