Cat Dun Cave straddles the border between Gia Hung commune in Ninh Binh province and Dong Tam commune - now Lac Thuy commune - in the former Hoa Binh province, today part of Phu Tho. It lies deep in the forest, shaded by towering trees and nestled at the foot of the Thung Chen mountain range. In front of the cave is a dry streambed, which only holds water during the rainy season.

Reaching Cat Dun Cave from Gia Hung requires traversing nearly ten kilometers of dense, muddy mountain trails. From Lac Thuy, the journey involves rough, rocky terrain stretching over a similar distance through remote forestland.

Locals from both provinces say the cave is named Cat Dun - “sand that rises” - because of a strange natural event. Every year in the eighth lunar month, sand pours out of the cave mouth. Then, by the third lunar month of the following year, it retreats back inside. The sand forms an impressive mound of up to several hundred cubic meters, yet it never exceeds 25 meters in length and 15 meters in width.

This phenomenon gave rise to a folk saying: “Tháng 8 xây ra, tháng 3 xây vào,” or “In the eighth month it rises out, in the third it pulls back.”

Legend holds that the appearance of the sand foretells the year’s fortune. If the sand forms a clean, golden mound shaped like a haystack, it signals a prosperous harvest. If it emerges as scattered black clumps, disease is likely to spread. And if the sand stretches into long ridges resembling levees or flood barriers, it portends heavy rains and floods.

Historical records in the Gia Vien Cultural History chronicle describe Cat Dun Cave as having two entrances. The first, located at the mountain’s base and facing southwest, is 4–5 meters wide. This is where the sand emerges and retreats. The second entrance, about 30 meters northwest of the first and elevated about 8 meters higher, faces west. The cave's interior spans from 3 to 15 meters wide with ceilings ranging from 2 to 20 meters high.

Captivated by the rare and inexplicable occurrence, villagers long ago built a shrine here to honor Chua Son Lam (the Lord of the Forest) and Duc Cao Son Thanh Hoang (the Mountain Spirit). The shrine, perched on a rocky ledge above the cave, remains standing to this day at the foot of the Thung Chen range.

Each year on the 15th day of the first lunar month, the Cat Dun Shrine Festival is held. It draws worshippers from the surrounding areas of Ninh Binh and the former Hoa Binh province (now Phu Tho), as well as visitors from far and wide. They come to offer incense, praying for favorable weather, prosperity, and peace.

A representative from the Department of Culture and Sports of Ninh Binh noted that although the phenomenon of the sand moving in and out of the cave has been observed for generations, no scientific explanation has yet been found. The mystery remains intact.