The Department of Homeland Security has a backlog of 600,000 known illegal immigrants, most of which they don't know the whereabouts of anymore according to this. 

To me, this just makes it even more obvious that rounding up the 12 million or more people in the U.S. illegally and sending them back to their home countries is an impossible fantasy.  It's a shame that we let it get this bad, but we should really be worrying about how to keep another 12 million from coming here illegally, not imagining unrealistic ways of getting the ones here out. 

As much as I hate to say it, there is going have to be some sort of amnesty for illegals already here.  It would be much cheaper to get these people in the system and taxed like everyone else, then to spend all the money it would take to kick them out or worse yet, allowing them to continue to recieve benefits they aren't paying for.