A 60 m deep single-shaft with a diameter of 50m formed by dissolution and collapse of 15-20 million years old limestone. This occurred after the limestone was uplifted from sea floor about 1.8 million years ago. Bottom of collapse-sinkhole connected with a horizontal passage that ends at grubug cave. Grubug cave, a single-shaft of about 90m, is the deepest vertical cave in java island. The phenomenon of grubug underground river and upward view from the bottom is very amazing.