The Persian Gulf War of 1991 and its aftermath caused severe environmental problems for Kuwait, releasing large quantities of oil into the environment and threatening the water supply. Kuwait has no renewable water resources and must rely on wells and desalination of sea water. The nation has some of the largest and most advanced desalination plants in the world, which provides much of its water. Kuwait's cities produce an average of0.9 million tons of solid waste per year. In 2001, one of Kuwait's mammal species and three of its bird species were endangered. The slender-billed curlew and hawksbill turtle are on the endangered list. The Sa'udi gazelle has become extinct in the wild.