Donald Trump said repeatedly during the campaign that he would build a wall on the southern border, but proposals being put together at DHS for a new barrier are focused on options that include a fence. One type of fence under consideration would be 18 feet tall and extend another 6 feet beneath the ground to prevent tunneling.

The final decision on the barrier will be up to Trump, and to the Congress which has to appropriate money to pay for it, but a concrete barrier may be the least likely outcome. A concrete wall would be more costly but CNN reports on another, more practical reason some would prefer a fence:

One senior U.S. Border Patrol official, speaking on the condition of anonymity, told CNN that it’s crucial to be able to see through a border barrier. “I’m not calling it a wall because we are talking about a fence that we can look through. That’s what we need.”

Another senior U.S. Border Patrol official told CNN, “I think fencing has worked for us in the past, but you never want to have a barrier in place that will obstruct your vision, that prevents you from seeing the other side of the border.”

CNN reports that one plan under serious consideration would involve 650 miles of upgraded fencing and another 175 miles of new fencing. However this plan would still only cover a total of 830 miles of the 2,000 mile long border.

The problem with the fence, of course, is that visibility works both ways. It may give border patrol a view of what is coming but it also allows people looking to cross illegally a chance to scout a location where border patrol isn’t waiting nearby. Also, an 18 foot fence can be climbed in a matter of seconds by healthy smugglers. In this clip shot by a Mexican reporter on the U.S. side of the border, Mexican smugglers have no problem bringing big backpacks over the fence and then heading back moments later after realizing they are being filmed: