Holy man-made caves, Batman! When a quarry here ceased operation in the late '20s, it left behind a rocky hole that soon became a go-to for Hollywood film shoots—in particular as the entrance to the Batcave in the '60s Batman series.
From Franklin Avenue, head north on Canyon Drive for 1.2 miles and you'll find a gravel parking lot on the right; there's also a larger paved lot just up the road. The trailhead sits between the two; from there it's only a quarter-mile walk to the cave.