The 1655-acre Circle X Ranch, a unit of the sprawling, federally administered Santa Monica Mountains National Recreation Area, is positively riddled with Tom Sawyer-esque hiking paths. Chief among those is the Grotto Trail, which leads to a rocky declivity featuring -- in this wet season at least -- a lively set of cascades. Circle X Ranch was for many years a Boy Scout retreat, so you'll be traveling in the footsteps of perhaps tens of thousands of youthful adventurers.
Note that this hike is almost entirely downhill on the way in, and uphill on the way back. Plan accordingly and bring along plenty of drinking water if the weather is warm.
You'll begin at the Circle X Ranch park office, on Yerba Buena Road 5.4 miles north of Pacific Coast Highway, or 5.5 miles west of Mulholland Highway. The most direct route from San Diego and L.A. takes you "north" (more precisely westward in the L.A. region) on the 101 Freeway to southbound Highway 23 near Thousand Oaks. Stay on southbound Highway 23 for seven miles to reach Mulholland Highway on the right. Take Mulholland 0.4 mile west to Little Sycamore Canyon Road on the right. Then drive the final 5.5 miles on Little Sycamore Canyon Road (which becomes Yerba Buena Road in a couple of miles when you cross the Ventura County line).
From the Circle X park office walk down to the group campground, where you can find and follow the Grotto Trail heading south down along a shady, seasonal creek. Keep heading downhill as you pass the Canyon View Trail intersecting on the left. Shortly afterward, you cross the creek at a point immediately above a 30-foot ledge which currently sports a modest waterfall. You then go uphill, gaining about 50 feet of elevation, and cross an open meadow offering fine views of Boney Mountain on the crest of the Santa Monica Mountains behind you, and a deep-cut gorge (the west fork of Arroyo Sequit) in front of you. Maintain your descent, which becomes sharper as you get closer to the bottom of the gorge.
Cross the Arroyo Sequit creek at the bottom, and continue downstream on a narrowing trail along the shaded east bank. Curve left when you reach a grove of fantastically twisted live oaks at the confluence of two stream forks. On the edge of this grove, an overflow pipe coming out of a tank discharges tepid spring water. Continue another 200 yards down along the now-lively brook to "The Grotto," a narrow, spooky constriction flanked by sheer volcanic-rock walls.
If your sense of balance is good, you can try clambering over a pile of massive, gray-colored boulders that have fallen from the canyon slopes into the canyon bottom. At one spot atop those boulders you can peer cautiously into a gloomy cavern, where the subterranean stream is more easily heard than seen. Water marks on the boulders above are evidence that this part of the gorge may support a two-tier stream in times of flood.
When you've had your fill of adventuring, return by the same route, uphill almost the whole way.