Sanmenhai is a cave in Poxin Village, Paoli Town, Fengshan County, Hechi City, Guangxi Province. It features a typical karst landform. There are three natural skylights in the cave, and there are passages that visitors can enter by boat. This gives visitors the mysterious feeling that there is a sea in the cave, with gates, and forms the basis for the cave's name, which means "three gate sea." Skylights are a new form of karst landform. There are seven parallel skylights in the shape of the Big Dipper in Sanmenhai, while only three of them are passable.