 The subsurface phenomenon includes karst caves consisting of no less than 119 caves that have stalactites and stalagmites , and all underground river flows.Because of the uniqueness of its ecosystem , the 1993 International Union of Speleology proposed that the Karst Area of the Sewu Mountains enter into one of the world's natural heritage.In the caves are also found fossils of ancient human bones from the old stone age about 1.8 million years ago along with stone tools for hunting..