Before he became president, Donald Trump promised to build an "impenetrable" barrier on the U.S.-Mexico border. "On Day One, we will begin working on an impenetrable, physical, tall, powerful, beautiful southern border wall," he said in August 2016.

But if that barrier gets built, it looks like it'll be pretty, well, penetrable.

At first, Trump wanted a concrete wall. Now putting aside for a moment the many arguments against constructing a wall, concrete would tough to breach—though not impossible. A February 2018 Customs and Border Protection report, which KPBS obtained in September, stated that all eight of the steel and concrete border wall types ordered by the president were vulnerable to breaching, though many of the specific breaching techniques were redacted.

In any case, Trump has recently shifted to calling for a steel-slatted barrier. That way, border agents on the U.S. side can see what's happening on the Mexican side. But it also means people with the right sawing equipment can cut through the wall. At least, that's what a photo of a breached steel slat prototype obtained by NBC News shows:

Dept. of Homeland Security testing of a steel slat prototype for border wall proved it could be cut through with a saw, according to a report by DHS. A photo obtained by @NBCNews shows the results of the test. https://t.co/SNxn6YneG9 pic.twitter.com/UP9EgHGxDx — NBC News (@NBCNews) January 10, 2019

NBC says the photo was taken after Marine Corps experts at "Pogo Row" (a testing location near the California-Mexico border) "were instructed to attempt to destroy the barriers with common tools."

San Diego Sector Border Patrol Chief Rodney Scott tells NBC that the prototypes tested at Pogo Row were not as big as the ones toured by Trump when he visited the border in March. But that shouldn't matter much: If they're made of the same material, they should be vulnerable to the same breaching technique, no matter how big.