The Denver Zoo is an 80-acre facility located in City Park of Denver with the concept that people should see animals at eye level in natural habitats to using naturalistic zoo enclosures rather than cages where possible without bars or fences.  Exhibits include Primate Panorama, featuring huge mesh tents and open areas for apes and monkeys. Predator Ridge, which has three separate areas through which animals are rotated so that their overlapping scents, provide environmental enrichment. Bear Mountain is a historic exhibit, on the National Register of Historic Places, one of the first natural-style zoo exhibits in North America, and the first to use simulated concrete rocks.