The Sierra Madre mountains rise from the Sonoran Desert where Arizona and New Mexico meet Mexico. It is one of the areas targeted by the Trump administration as it rushes to complete its promised.
Construction of the last 35 miles of the wall in Arizona is now underway in the rugged mountains. And it is challenging as well as controversial, because of the pristine land the wall is cutting through.
"It's a huge richness of biodiversity, of culture and relationship between the two nations,"Jose Manuel Perez told CBS News' Michelle Miller. "We are losing that with this wall."
Construction requires roads to be carved out of mountainsides so workers, machines and materials can sink 30-foot high steel beams into concrete. Costs have been reported at $41 million per mile, more than any other stretch of the 738-mile project.
Perez, however, sees a higher cost.
While the wall is intended to stop people, it also stops migrating animals. Perez said it is the "biggest corridor" for big mammals like mountain lions and jaguars.
"They don't recognize any boundaries. They've been here forever," said Perez, who runs a land preservation organization, Cuenca Los Ojos.
His border cameras capture images of animals in the area, and his group is one of several that are dedicated to preserving the area on both sides of the border.
The environmental push was started by a local landowner, Valar Clark, who moved to the Sierra Madre region 40 years ago from New York City. She learned barren land can flourish if rocks were piled up to retain scarce water. Her crew made tens of thousands of them.
Later, she bought 130,000 acres on the Mexico side as well, for Cuenca Los Ojos to care for. Clark also convinced reluctant ranchers to join the conservation effort.
Now, however, decades of work is being blown up — literally. And unlike other building projects that require review under the National Environmental Policy and Endangered Species Act, some protections blew up, too, after the September 11 attacks. The border wall is a national security project.
President-elect Joe Biden wants to stop the building and leave motion sensors already there to detect human movement, but the building for now is moving ahead — conquering and transforming a landscape, possibly forever.
CBS News has reached out to multiple federal agencies for comment on the wall's environmental impact.