is the tomb of Sultan Paduka Pahala of Sulu (Philippines), who died in Dezhou on his return journey from a visit to the Yongle Emperor in 1417. The tomb is well preserved and has been declared a national heritage site. Descendants of the sultan's Muslim followers still live in Dezhou today, and are classified as the Hui minority. One of Dezhou's county Lingxian used to be a big county in China in history, when it was called Pingyuan County. Now part of the ancient city wall of Tang Dynasty still exists in the south of the region. Before the Three Kingdoms formed, one of the three emperors Liu Bei used to be the chief of the county, together with his fellows Guan Yu and Zhang Fei. In addition, it is the hometown of Dongfang Shuo, the most well-known adviser during the reign of Emperor Wu of Han ; In Tang Dynasty, a major figure of Chinese calligraphy, Yan Zhenqing, once took office as the administrator of the county. Today, a memorial hall for the two historic figures is built in the People's Park of Lingxian, where a lot of materials of Dongfang Shuo and stone inscriptions of Yan Zhenqing are preserved. For tourist attractions, there is also a famous temple in Qingyun County. It's called Haidao Jinshan Temple, which is one of the biggest centers of Buddhism in Northern China. The most attractive scene is the underground aisle where the portrait of the hell is presented using high technology. Industry A new industrial zone hailed as the "Solar Valley" is being built for experimenting with clean-energy urban projects and massive use of household utilities such as solar-powered water-heaters. The Washington Post describes Dezhou's Solar Valley as the "clean-tech version of Silicon Valley". Nowadays one of the biggest and most famous industries in Dezhou is solar energy industry, with two main corporations included—Himin Group () and its partner Ecco Solar Group (). Dezhou increased its international reputation when it was selected to follow previous hosts, Daegu, South Korea (2004), Oxford, UK (2006) and Adelaide, Australia (2008) as host of the 2010 International Solar City Congress. Himin Group has developed into the world's largest solar water heater manufacturer and is also discovering new areas such as photoelectricity. Greenpeace China cited Dezhou in May 2009 as an example of how renewable energy can become a more common reality throughout the world. Dezhou also