Thirty-one species of cacti live in the starkly beautiful Organ Pipe Cactus National Monument in the Sonoran Desert, along with bighorn sheep, pronghorns, javelinas, kangaroo rats, rare pupfish and all kinds of reptiles and birds. But the green desert is also home to a 30-mile stretch of America’s steel border wall and warning signs had this traveler anxious that “smuggling and illegal immigration may be encountered in this area.”