“Chain link fence and constant patrol!”
In practice, Border Patrol has found that they have to repair breaches in fencing virtually every day, and some flaps can be artfully cut, so they are hard to see. Such breaches can sometimes be used for a long time before they are noticed. Even small kids can usually scrape away enough dirt to wiggle under a fence in a short time.
It is a major operation to bust through heavy steel, concrete-filled bollards - they can stop a speeding truck. It is basically the end of maintenance, once bollards are installed. With two foot thick concrete footings extending six or more feet deep, tunneling is only for the pros.
a patrol every mile for 700 miles 24/7 would cost less than $250 mil.