100% border security is not possible. If we could get up to 97% or greater security, that would be satisfactory.
Sonar or similar technology might detect some tunnels, and then we'd have to depend on local law enforcement and the BP to detect strange activity that might indicate a tunnel exit on the US side.
Doubt we'd ever get any help from the Mexican side.
One thing that has to go along with any effective plan is we have to up the ante on crossers. It needs to be a very serious offense to cross the border illegally. If it was a serious crime and treated that way instead of a misdemeanor that is seldom prosecuted that would cut down considerably on those trying to cross. Resources could be aimed at the remaining die hards.
We have to remove jobs and all prizes on this side, deport illegals, strictly enforce all of our laws. The border would be much easier to secure.
It all has to be a package deal, none of this piecemeal hit and miss political mess we have had for so long.
People say the border cannot be secure and illegals cannot be deported. That is not true but the will has to be there.