I'm not saying that I am all that much against a fence; however, that alone, is NOT the answer; neither is pulling our troops home and stationing them on the Southern border.
"And what happens, after that fence is built and the tunneling becomes more rampant?"
What happens?
Some illegals get through.
There will be border patrols, and aerial patrols.
Tunnelling across a triple fence line is not easy, and once you open a tunnel and people start to move, it doesn't stay open very long.
Sure, some people will always get through. But not 2 million a year.