Placed in reserve in 1835 (by the then Mexican government of Alta California), the park's site is one of the oldest in the United States dedicated to public recreational use (1868). The park hosted the 1915-1916 Panama-California Exposition and 1935-1936 California Pacific International Exposition, both of which left architectural landmarks. The park and its historic Exposition buildings were declared a National Historic Landmark District and also placed on the National Register in 1977. After 1962, the only 1915 Exposition buildings on El Prado were the California Building and its annexes, the House of Charm, the House of Hospitality, the Botanical Building, the Electric Building (rebuilt as Casa de Balboa in 1981) and the Food and Beverage Building (rebuilt as Casa del Prado, 1971). In addition to open space areas, natural vegetation green belts, gardens and walking paths, the park contains museums, several theaters, and the world-famous San Diego Zoo.