Over the years, that first wall, now splashed with murals, has metastasized.

A second fence stretches behind most of it, and between the two lies a no man’s land of cameras, sensors and floodlights.

This year the border agency began to replace the old metal wall. The new sections, between 18 and 30 feet high, are built of closely spaced steel posts topped with a steel plate designed to deter climbers.

Despite these changes, ask almost anyone in Tijuana about the wall, or “la línea,” and you are likely to be met with a shrug: The wall is always present, but not a preoccupation.

“We live very comfortably here,” said Elizabeth Quintana, 73, who runs a small restaurant from her house on a dead-end street that runs into the wall.

When she moved to Tijuana’s Libertad neighborhood in 1972, the border was just a few shin-high cement markers. Her only complaint about the giant steel bars that mark the line these days: To install them, she said, “they pulled up all the trees.”

Daily life in Tijuana is defined less by the wall as an impenetrable obstacle than by the ebb and flow of movement across it — or, for many, the distant hope for such a journey.