Taipei lies on the east bank of the Tan-shui river between the mouths of two of its tributaries, the Hsin-tien and the Chi-lung. These rivers drain a fertile and humid basin surrounded by mountains on all sides but the northwest, where the Tan-shui flows into the Taiwan Straits facing the mainland. It was here, near the harbors of the Tan-shui estuary, site of the present-day town of Tan-shui, that the first Chinese settlements in northern Taiwan were established during the first half of the seventeenth century, and perhaps earlier as pirate bases.