Ondal Cave is located underground at the foot of Seongsan Mountain, where Ondalsanseong Fortress is situated. It is a natural limestone cave that is estimated to have begun forming around 450 million years ago. The total 760 meter-long cave covers an area of 349,485 square meters. The cave mouth is two meters high, while the cave itself is 5 to 10 meters high and about 5 meters wide. The cave features stairway structure and contains an abundance of beautiful stalagmites and stalactites. The average temperature of the cave maintains 14 degrees Celsius in summer and 16 degrees Celsius in winter, making it a perfect all-year-round attraction. The cave is rich in underground water resources and there is a 0.8 to 1 meter-deep cave river flowing at the bottom of the cave floor. The cave also has over 40 small ponds that are inhabited by a variety of living organisms like fish and insects. Situated on top of the mountain peak to the left of the cave is Ondalsanseong Fortress. The fortress has been preserved in its original form throughout some 1,400 years and shows the advanced building skills of Goguryeo Era (37 BC-668 AD). The annual Ondal Culture Festival is held every October around Ondal Cave and Ondalsanseong Fortress in memory of the legendary love story between General Ondal and Princess Pyeonggang. Other attractions in the area include a theme park and Jangseung (Korean traditional totem poles) Park.