Passage: Located south of Yan Mountain, and north of the Bohai Sea, for centuries the pass guarded the narrow passage between Northeast and Central East China. Both the Northern Qi Dynasty and the Tang Dynasty constructed passes here. In 1381, Ming general Xu Da constructed the present pass, which was named Shanhaiguan (literally "mountain-sea-pass") because of its position between the mountains and the sea. In the late 16th century, Ming general Qi Jiguang began fortification and construction of a military city around the pass, building cities and forts to the east, south and north, making it one of the most heavily fortified passes in China. Today it is one of the best preserved passes in the Great Wall.

Question: Why is the pass between Yan and Bohai called Shanhaiguan?
because of its position between the mountains and the sea