Earlier this month, they and a group of friends spent nearly a week stringing a single, 2,800-foot (853-meter)-long line from Taft Point west across a series of gulleys that plunge 1,600 feet (488 meters). Moises and Daniel Monterrubio, brothers who are training to be rope-access technicians, had been thinking about crossing that void for a year. Highlining is high-altitude slacklining, in which a narrow strip of strong, nylon webbing — usually an inch wide and a few millimeters thick — is strung between two anchor points and serves as a kind of balance beam. Completing a line means carefully heel-toeing...