Fangshan is home to the largest karst cave group in northern China. Its geological structure is unique. Formed by karst processes over a long period of time, the cave features hard stone walls with unique textures, as if telling the vicissitudes of time. Because the rock walls are shaped like oysters, it is also called the "Oyster Cave"; at the same time, since the cave has a constant temperature all year round, being warm in winter and cool in summer, like a natural refrigerator, it is also known as the "Refrigerator Cave".