A 40 meters deep single – shaft with a diameter of 50 meters formed by dissolution and collapse of 15-2 million years old limestone. This occurred after the limestone was uplifted from the sea floor about 1,8 million years ago. Bottom of collapse – sinkhole connected with a horizontal passage that ends at Grubug cave. Grubug cave is a single shaft of about 90 meters, is the deepest vertical cave in java island. The phenomenon of Grubug underground river and upward view from the bottom is very amazing.