Popunder ads are also extremely annoying became some methods of getting them to stay under the current browser window cause EVERYTHING to stay under. This means that if you go to a web page with popunder ads, while it and its ads are loading you will be forced to sit there and watch it, as switching to any other window will force the ad-spawning page to instantly pop back up. I have never looked into this, but I believe it's done with JavaScript's onBlur event and window.focus() method. Why a script can force a browser window into focus at will is beyond me.