Santa Monica Mountains National Recreation Area 30000 Mulholland Hwy, Agoura Hills, CA91301 Containing more than 154,000 acres of land, the Santa Monica Mountains National Recreation Area is the largest urban…More national park in the United States. The park was established on November 10, 1978, following a successful campaign by local conservationists. Recreational opportunities abound at the site, where popular activities including biking, birding, camping, horseback riding and hiking. The Backbone Trail System runs for nearly 70 miles across the length of the park. For more information, visit the park's Web site.