According to legend, this cave was used as a passage between the underworld and the human world. The cave's earthy structure resembles the mythical underwater city of the Naga. Inside, it is consistently damp with a flowing stream throughout the year. There are stone pillars that appear to have been deliberately placed. If anyone wishes to experience the underworld up close, it is highly recommended to contact a guide, and unauthorized entry is strictly prohibited. For inquiries, please call: 09 3067 7959. For travel directions, take National Highway 211 from Nong Khai, following the Mekong River for approximately 96 kilometers. When you reach Ban Dong Tong in the Phathang sub-district, there will be a sign on the left indicating the way to Wat Tham Si Mongkol, which is 14 kilometers away.