In the unofficial sport of parkour, there are no teams or regulations, just acrobatically inclined individuals traveling a short distance in a straight line — the more wall-scaling and rooftop leaping, the better. For the traceurs who use public stairways, ledges, and lampposts in their quest for a mind-over-matter outlook and killer calves, the world is their practice gym. Problem is, a lot of the world is private property. While some have negotiated unofficial practicing rights with nightwatchmen, those concerned with the letter of the law favor the bunkers in the Presidio, a veritable jackpot of ladders, rails, and concrete surfaces of enough heights and angles to confound a Cubist. Beginners practice running up short, sloped inclines, while experts can perfect their "kong vaults" and "cat balances" over waist-high ledges and second-story rails. And just about anyone can find two walls placed just far enough apart for precision jumps that make you feel like James Bond. One warning: Although park officials haven't banned parkour, they do officially discourage anything potentially dangerous. But such is the dilemma of the sport that lives free of corporate sponsorship and health insurance.