U.S. President Donald Trump, through a recent executive order, is keeping his campaign pledge to build an “impenetrable, physical, tall, powerful, beautiful, southern wall” between the U.S. and Mexico. It’s not clear to what extent the project will replace barriers that already exist between the two countries.

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A fence already covers one-third of the border

About one-third of the 3,200-kilometre border — which stretches from the Pacific to the Gulf of Mexico — already has various kinds of fencing, thanks to the 2006 Secure Fence Act signed by then-president George W. Bush. The wall starts in San Diego. Design depends on geography and climate, and ranges from metal barriers with pickets to ground sensors, cameras and drones.

The San Diego border is heavily fortified

The San Diego area, one of the most heavily fortified, has 75 kilometres of primary fencing. The barriers are about 5.5 metres high, with pickets to impede pedestrians. There is also 22 kilometres of secondary fencing — 4.5 metres high and built with horizontal rails in the form of steel tubes and fence fabric that is either mesh or perforated metal sheeting.

A canyon along the border was filled in

Smuggler’s Gulch, a canyon between San Diego and Tijuana about three kilometres from the Pacific Ocean, has been filled in with a structure resembling an earthen dam. There is also a triple-thickness border fence topped with razor wire, flood lights, remote sensors and cameras to deter nighttime crossings.

One existing wall failed to thwart drug smugglers

Although the sister cities of Calexico, Calif., and Mexicali, Mexico are separated by a wall, in April 2015, U.S. Border Patrol agents seized more than 30 kilograms of methamphetamine coming over the border. In the process, they found how smugglers were getting around the wall between Calexico and Mexicali — they had built a tunnel.

A wall would further divide traditional Native American lands

Tohono O’odham Nation reservation in Arizona has said no to Trump’s plan. The tribe already struggles with the steel-post fence that currently lines most of the reservation’s southern border. A wall, they say, would further divide and militarize traditional lands. Trump has proposed a congressional bill that would override tribal resistance.

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In Texas, natural barriers already exist

Texas is the state with the least amount of fencing because of the Rio Grande, and because large swaths of its southern border are mountainous, making it difficult to cross. Treaties with Mexico prohibit the construction of barriers within the Rio Grande’s flood plains. Some border land in Texas is privately owned.

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