Limestone has two properties that make it susceptible to cave formation.
Limestones have joints running at right angles to each other.
Water seeping through the soil also dissolves substances such as hummic acid, giving the water a greater solutional action. Water then enters the joints, and gradually the joints are widened. Joints which are widened when completely submerged are called phreatic. Solutional activity takes place equally on each side of the widening passage, and phreatic passages have a typically circular cross section. Once water starts flowing through the passages, then mechanical abrasion takes place accelerating passage growth.