Silesia is a historical region of Central Europe that lies mostly within Poland, with small parts in the Czech Republic and Germany. Its area is approximately 40,000 km², and the population is estimated at 8,000,000. Silesia is split into two main subregions, Lower Silesia in the west and Upper Silesia in the east. Silesia has a diverse culture, including architecture, costumes, cuisine, traditions, and the Silesian language. The largest city of the region is Wrocław. Silesia is situated along the Oder River, with the Sudeten Mountains extending across the southern border. The region contains many historical landmarks and UNESCO World Heritage Sites. It is also rich in mineral and natural resources, and includes several important industrial areas. The largest city and Lower Silesia's capital is Wrocław; the historic capital of Upper Silesia is Opole. The biggest metropolitan area is the Katowice metropolitan area, the centre of which is Katowice. Parts of the Czech city of Ostrava and the German city of Görlitz are within Silesia's borders.