Sandstone Retreat, officially the Sandstone Foundation for Community Systems Research, was a clothing-optional, open sexuality resort for swingers located at Sandstone Ranch, a 15-acre (6.1 ha) estate in the Santa Monica Mountains overlooking Malibu and the Pacific Ocean. The retreat offered members and guests over the age of 18 the resources of a spa with the addition of several large open communal sleeping areas, both indoor and outdoor, and communal bathrooms. Prospective members were interviewed on a daytime visit to determine suitability.