SAN MARCOS -- Dust off your hiking shoes, saddle up your horse and get ready to explore uncharted territory during the 15th annual San Marcos Trails Day on Saturday.

The free event is part of the city's fall series of "Discover San Marcos" hikes, held on the first Saturday of every month except for holidays. The series encourages residents to enjoy the San Marcos trail system, and one hike during the season allows people to explore trails not usually open to the public, said Bill Schramm, community services director for the city.

"The excitement is in having the opportunity to see some backcountry of San Marcos," said Schramm. "When you live in a suburban environment like San Marcos, it's good to get out and see birds singing, fish jumping, and experience the quiet, beautiful moments in nature."

Hikers can explore South Lake, Double Peak and the surrounding Valley of Discovery, starting from the staging location at the south end of Santa Barbara Street off Craven Road at 9 a.m.

Participants can choose from a 4- or 6-mile hike around South Lake, a route open to the public only for this event, while equestrians can ride on 6 miles of rural scenic trails. In the South Lake area, hikers will get a rare glimpse of the proposed 44-acre South Lake Park and a tour of the 300-acre Hanson Aggregates' rock quarry.

"It's neat because people get to see how they crush rocks, and all the machinery involved," Susan La Joie, director of Jack's Pond Nature Center, said recently.

The 6-mile, round-trip hike to the top of Double Peak is more challenging, La Joie said, and offers hikers a chance to stand 1,600 feet above sea level on the second-highest peak in the Cerro de las Posas range.

"Usually, hikers are divided into slow, medium and fast-paced groups," said Schramm. "Those feeling energetic will get a great overview of the ocean from the ridgeline."

Formerly known as The Back Country Ramble, the city began Trails Day 15 years ago to encourage North County residents to make use of the trail system by exploring the flora and fauna of the area.

The city has 40 miles of trails that link schools, parks and neighborhoods to nearby Carlsbad and 72 planned miles of trails that will eventually connect to Vista, Escondido and Oceanside, said Schramm. As the community grows, it is the responsibility of developers to build trails connecting to the new areas, he said.

"It's a domino effect," he said. "These are loops families and individuals can take to go fun places like the university and Restaurant Row. In a collegiate town, it's an alternative method to a vehicle for getting around."

Registration is free and includes trail snacks, an event packet and trail guides. Check-in begins at 8:30 a.m.

For more information, call (760) 744-9000, Ext. 3505.

Contact staff writer Noelle Ibrahim at (760) 761-4404 or nibrahim@nctimes.com. Comment at nctimes.com.