After turning off the West Ajo Highway, 20 miles west of Tucson, a car full of volunteers heads down a dirt track, carefully traversing the wide fractures in the road en route to plant the first cross of the day. The road is flanked on each side by rusted four-barred fences, behind which lie a couple of buildings with corrugated iron roofs.
The car is quiet, apart from the spade, bucket and cross rolling around in the trunk.
The car parks and the riders get out and stand in a circle. David, one of the volunteers, glances back and forth between a piece of white paper in his left hand and a GPS satellite phone in the other, listening for the distant beep every time the system acknowledges a coordinate.
Still looking at the GPS, David sets off slowly behind the others. Diane holds a pick axe; Alvaro Enciso, the artist who leads this project, drags a spade; Haley lugs a large water container; Alyssa carries a wooden cross and Peter leans gently to his left side to counteract the heavy bucket of cement in his right hand.