88m from the ground, the ancient cave was where the people of Hoa Lu citadel set up the sky to pray for the rain and wind. Today, this place is known as a fairy -tale place when fully converging "hell - earth - heaven" in the ancient capital. From the upper door of the cave, it is possible to observe the whole scene of Thung Nham mountains with coincidental mountains, the point in which is stilt houses to create a beautiful and vivid picture.