In 2015, the IWF identified 743 websites that "provide a secret route to child sexual abuse content," by showing only legal content when accessed via a browser, but hosting illegal materials out of sight behind a maze of links. That's more than double the number from just 18 months earlier. About 21 percent of those sites were set up to profit from the trade of those illegal materials.

The disturbing trend should be a wake-up call to everyone, as even those who only seek out and view legal content could potentially find themselves with a suspicious cookie trail leading back to a website they didn't know was serving as a front for illegal activity.

"It's extremely significant," IWF's executive director Fred Langford explained to The Guardian. "Some people may think they're going to legal adult content but they unfortunately have picked up the cookies on the way that means that they are served the child sexual abuse material. There's a huge risk there."

As watchdog groups, law enforcement agencies and tech companies ramp up their efforts to detect and remove child pornography from the internet, their increasingly difficult work also reveals a harsh truth: The problem is always bigger than it initially appears. In the two years since UK Prime Minister David Cameron allowed the IWF to proactively seek out illegal imagery, the number of pictures and videos it has removed has more than quadrupled. Overall, the IWF found 68,092 cases of confirmed, illegal child sexual abuse images in 2015.