sanmenhai karst windows are the most beautiful and largest of the karst windows in the geopark. they are located at the downstream end of the poxin underground river, at ~460 m asl four karst windows 80~100 m in diameter and 50~120 m high are located along a 690 m stretch of river passage. the area of the underground river exposed by the karst windows totals 7.2 ha.