If you're a hiker or mountain biker who visits South Mountain and you're not into climbing, you may have looked in bewilderment at some point at people walking through the bush with odd-looking backpacks. No, they're not terrorists going to plant IEDs to take out the occasional mountain lion in this 16,000-acre municipal park. They're rock climbers with portable crash pads heading to the Valley's single best concentration of tough boulders for bouldering. Ninety-five routes are listed on MountainProject.com, but be sure to pick up the Marty Karabin pamphlet on the place for $5 at your local outdoor store. This is not a site for beginners. If you're sketchy at easy bouldering grades like V0 and V1, you won't love it here, and you might twist an ankle in the rocky landings, but climbers with skills and crash pads will find hours of entertainment and the inevitable bloody flapper on a hand. Get to the address above early, park, and walk the dirt road west. At its end, take the arroyo southwest until you hit the boulders described in the guides. Though Arizona has some terrific climbing areas, good rock is tough to come by in the central Valley. South Mountain bouldering helps make this place livable.