A category unto itself, like oolong, green or white, Pu-erh could well be called China's mystery tea. It has been produced in Yunnan from ancient times and shipped from the market town of Pu-erh until eventually the tea itself acquired this name outside Yunnan. Pu-erh is made tea, either black, green or white, which is then moistened or sprayed with an unknown bacterium and allowed to undergo a sort of secondary fermentation. It may be compressed into many shapes or left loose and, unique among teas, it actually improves with age. The best Pu-erh comes from the area called the Six Famous Mountains in Xishuangbanna region.