Let me start by saying I hate pop-unders. They clutter up peoples desktops and generally annoy users. Unfortunately they are one of the most effective forms of web-based advertising at the moment, although I'm not sure why. I've had a few questions over recent months about how to implement pop-unders so here it is.
That will cause the popped-up window to sink behind whatever opened it.
Note that I accept no responsibility for the complaints you'll get from users if you start doing this. You're on your own!