University of Leeds physical education teacher Don Robinson took his pupils climbing and spelunking near Sheffield, England, three seasons out of the year. However, the winter months away from the rock took their toll: Robinson’s climbers got hurt the most when they returned to rock after a months-long hiatus. To facilitate year-round climbing, he found rocks that simulated all different outdoor features—slopers, crimps, underclings—and, in 1964, built the world’s first indoor climbing wall out of found rock. Gaps in the stonework allowed indoor climbers to practice jamming. Nowadays, indoor climbing walls are primarily being built out of ¾-inch plywood, and holds are manufactured from urethane or resin.