In recent years, the popularity of cider has witnessed an international revival. Gone are the days when cider was classed as a "curiosity" drink. Cider has quickly become a part of the mainstream beverage industry. Most cider is made from fermented apple juice. Natural cider has nothing added and relies on the wild yeast present in the apples for fermentation. For mass-produced ciders, a yeast culture is added in order to achieve consistency. Although much of today's cider is produced from apple concentrate, many traditional cider makers use only cider apples, cultivated specifically for the purpose. Both traditional and mass-market ciders are available carbonated or still and range in taste from the bone dry to the extremely sweet. In Europe, "cider" refers to fermented apple juice that contains varying levels of alcohol. In the USA, fermented apple juice is known as "hard cider," while unfermented, freshly expressed juice is called "sweet cider."