This past summer, Google started noticing ads for vacation homes that didn't exist. The ads themselves weren't fishy — there was no malware or counterfeit goods — but there was something suspicious behind them. The photos were all pulled from other listings or from stock photo sites, and didn't match with the addresses. The ads were real, but the homes weren't. The point was to convince users that they were real for just long enough to get a deposit, at which point the company could safely disappear. Once Google got wise to the scam, they cracked down, poring through the system for any rental deals that might be bogus.

For Google's Bad Ads team, it's a common story. The summer before, a similar scheme had played out with Chinese car ads, to similar effect. This morning, the group released its 2014 year in review, running down 2014's most popular schemes. All told, the group banned more than 214,000 advertisers over the course of the year, disabling more than 500 million bad ads. It's a sliver of the total traffic on AdWords, but active investigation is a necessary step to maintain the system's reputation, and Google attacks it with the usual pragmatic engineering ethos.

Another common scam came with this summer's Ebola outbreak. When the first US cases of the disease were reported, search traffic for "ebola" went through the roof, attracting scammers. Shortly afterwards, the first phony ebola treatments surfaced, selling discredited treatments for the disease. Since AdWords has a blanket policy against misleading claims, the ads were easy to weed out once identified.

Because of the immense scale involved, bad ads are typically flagged through algorithms and then investigated in-person. Google has a number of quirkily named tools designed specifically for that purpose — the index of ads is known as Beaker, while the analytics tool is called Bunsen — but the key moment in each case is noticing something's not right. "Once we have a lead on where we think there's going to be problems, we can use those tools to operate on a lot of different kinds of badness," said Ads Engineering director Vikaram Gupta.