Be careful out there. There are a few kids who filter out of the College Park East on a little 110 and an old minitrail. They set up a few jumps and berms in the area but have poor attention to trail details like logs and thick brush in the impact zone. They do know how to hide the holes in the chain link fence (and the holes only fit a minibike, so leave um some fun). The big field south of Willow is accessed from the Nature Center, and observed on a regular basis by the bird & coyote vs. squirrel fans. It's not worth it unless you want to get caught. Gum Grove park single track used to be good for one or two blasts in the late 80's, before Seal Beach's finest would park at the Hellman oil field gate west of their Station. I doubt a mountain bike could ride without complaint today. Your best bet for a photo is the north side of Signal Hill (access off either end of the Promenade). Unfortunately it's almost all gravel roads, scenic but no challenge. When the Roughnecks are working they will call it in (so know your exit options).