What keeps the enemy from simply building tunnels beneath a wall, especially in sparsely guarded areas?
They will try and be successful in some cases. But tunnels are time consuming and will concentrate the crossings which means they will be easier to find and seal with a lot less people crossing illegaly than we now have with no major impediments. Using technology we now have such as manned aircraft, drones, space telescopes, etc we can map sections and analyist will be able to spot such attempts. JMO.