Zhoushan is a prefecture-level city in northeastern Zhejiang Province of Eastern China. The only prefecture-level city of the People's Republic of China consisting solely of islands, it lies across the mouth of the Hangzhou Bay, and is separated from the mainland by a narrow body of water. The archipelago was inhabited 6,000 years ago during the Neolithic by people of the Hemudu culture. The Zhoushan Archipelago, comprising 1,390 islands and 3,306 reefs, is located outside Hangzhou Bay. It is the largest archipelago of China (not including South China Sea Islands). Among these islands, 103 are inhabited all year round, 58 are larger than 1 sq km; (these make up 96.9% of the archipelago land area), and only 15 have populations over 10,000.