The San Diego Zoo is a zoo in Balboa Park, San Diego, California, housing 4000 animals of more than 650 species and subspecies on 100 acres (40 ha) of Balboa Park leased from the City of San Diego.[7][8] Its parent organization, San Diego Zoo Wildlife Alliance, is a private nonprofit conservation organization, and has one of the largest[better source needed] zoological membership associations in the world, with more than 250,000 member households and 130,000 child memberships, representing more than a half million people.[9] The San Diego Zoo was a pioneer in the concept of open-air, cageless exhibits that recreate natural animal habitats.[10] For decades, the zoo housed and successfully bred giant pandas,[11] with the largest giant panda population outside China, before the pandas were repatriated to China in 2019.[12] With more than 4 million visitors in 2018,[3] San Diego Zoo is the most visited zoo in the United States.[13][14] Travelers have also cited it as one of the best zoos in the world.[15][16] The San Diego Zoo is an accredited member of the Association of Zoos and Aquariums (AZA), the American Alliance of Museums (AAM), and the World Association of Zoos and Aquariums (WAZA). The San Diego Zoo Wildlife Alliance also operates the San Diego Zoo Safari Park.