Deep enough so he can't dig it out.
Then scan every one sneaking back in to find the repeats.
Put them in jail for 10 years and bill Mexico for their incarceration.
Then scan every one sneaking back in to find the repeats.
That only works when they come back through a Port of Entry manned by immigration officers? Do you have a freaking clue what percentage of illegals do that? Try less than one in a thousand.
Problem is most simply walk back in, so who would/could scan them??
The fact that most illegals simply walk in is the very reason we need a major fence/wall barrier on the border. There is no way to check out the ones that sneak in, either the first time or repeated times unless they are of the few that get caught.
We simply must keep them out to begin with. No plan to deal with illegals or other border issues will work while the border is still wide open.