Pop-Under Problem

Advertisers have gotten smarter and figured out ways to bypass pop-ups blockers with the pop-under ads. They aren't new but before the last few months Safari and Mozilla could easily block pop-under ads. Now, I'm getting pop-under ads each session with Safari and Mozilla. Why is this happening now? What's different about these pop-under ads that they're defeating my browsers' blockers? Is anybody else having this problem?