To report a multipart visual series on border barriers around the globe, we drove the entire length of the U.S.-Mexico border — from Brownsville, Tex., to San Diego, with crossings into Mexico scattered throughout — talking to locals and experts about President Trump’s promise to build the wall.

Trump’s action to start construction leaves unclear whether the administration will build a wall along the entire border or just in certain areas.

Below are five notable challenges to building the wall that we observed along the journey.