Several populations have now been released into the wild. A cooperative venture between the Zoological Society of London and Mongolian scientists has resulted in successful reintroduction of these horses from zoos into their natural habitat in Mongolia. In 1992, 16 horses were released into the wild in Mongolia, followed by additional animals later on. One of the areas to which they were reintroduced became Khustain Nuruu National Park in 1998. Another reintroduction site is Great Gobi B Strictly Protected Area, located at the fringes of the Gobi desert. Lastly, in 2004 and 2005, 22 horses were released by the Association Takh to a third reintroduction site in the buffer zone of the Khar Us Nuur National Park, in the northern edge of the Gobi ecoregion. In the winter of 2009–2010, one of the worst "dzud" or snowy winter conditions ever hit Mongolia. The population of Przewalski's horse in the Great Gobi B SPA was drastically affected, providing clear evidence of the risks associated with reintroducing small and sequestered species in unpredictable and unfamiliar environments. , an estimated total of almost 400 horses existed in three free-ranging populations in the wild. Since 2011, Prague Zoo has transported 35 horses to Mongolia in eight rounds, in cooperation with partners (Czech Air Force, European Breeding Programme for Przewalski's Horses, Association pour de cheval du Przewalski: Takh, Czech Development Agency, Czech Embassy in Mongolia and others) and it plans to continue to return horses to the wild in the future. In the framework of the project Return of the Wild Horses, it sustains its activities by supporting local inhabitants. The zoo has the longest uninterrupted history of breeding Przewalski's horses in the world and keeps the studbook of this species.