Not that you're using pop-unders on your website, but if you were (maybe) thinking about it (perhaps because of their proven performance) you should at least know Google's stance on the matter if you were looking to monetize those ad units through AdSense.
Google has actually long had policies in place that define where Google AdSense ads can appear and how they should be implemented, but while most experts agree that popups and popunders provide a rather poor user experience, websites still seem to use them - and it's because they work (almost too well in some cases).
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It's likely for that very reason that Google
recently indicated that it is clarifying its policies around the use of pop-ups and pop-unders that try to monetize through AdSense units, you know, to help remove any ambiguity on the matter.
Google wants to make it very clear - it is no longer permitting the placement of Google ads on pages that are loaded as a pop-up or pop-under.
John Brown, Google AdSense Head of Publisher Policy Communication also indicated that Google does not permit AdSense ads on any site that contains or triggers pop-unders, regardless of whether Google ads are shown in the pop-unders.
So, to be really, really clear: don't use AdSense if you've ever even thought about integrating pop-up or pop-under windows on your website - but feel free to integrate some
nice native ads within your content stream.