See post 103, gracias.
Tunnels are used for extremely high value missions such as smuggling heroin and cocaine, and we have been having great success at finding and closing them down. There is no way in the real world that 100s of thousands of illegal aliens are going to pay for new tunnels, and sneak under the border. That is a childish fantasy.
And even if we somehow prevent that, all any Mexican needs to do is catch a plane to Canada and enter from the north. For what they pay coyotes to smuggle them, I don't know why they are not doing that already. Maybe they are.
Funny thing about flying from Mexico to Canada. They need visas, it costs $$, and the numbers going in and out of Canada are easily monitored. Somehow, I think we'd notice a million Mexicans a year flying one-way into Canaday. Again, a childish idea.
And then there are thousands of miles of unguarded coastline where illegals could simply unload and walk freely onshore.
Unload from what? No way can boat smuggling take up the slack for the million plus who try to walk over the open border every year. Noticed any Haitians or Cubans deluging Florida lately? No, you have not. It's easy to stop them by sea in the numbers we're talking about, that is, the tens of thousands a month who simply walk over an open border.