Before 1886, the 904-foot hill behind the UCSF Medical Center then known as Mount Parnassus was covered in low brush and chaparral, just like the rest of San Francisco. Mayor Adolph Sutro celebrated Arbor Day that year by planting the hill with thousands of blue-gum eucalyptus trees, that era's wonder plant. Decades later, laborers with President Franklin D. Roosevelt's Works Progress Administration cut a network of hiking trails through what had become a dense thicket of eucalyptus, blackberry, elderberry, soaproot, and ferns. (Later still came the giant Sutro Tower radio antenna the hill is now known for.) A half-dozen years ago, Rotary Club volunteers cleared a few acres on the hill's crest and planted a garden of native low-lying plants, creating a parklike vignette of the hill's pre-Sutro days. During the last couple of years, more volunteers have been putting the final touches on what may as well be called San Francisco's secret manmade wilderness preserve, clearing the old WPA trails so that hikers, joggers, and families can spend an hour or so wandering through the old trail labyrinth without sensing the city beside them.