Tham Din Phiang, also referred to as Tham Phaya Naak, is a tunnel cave within the compound of Wat Tham Sri Mongkhon (fig.) in Nong Khai. The grotto is said to be the abode of the naga and a golden statue of a seven-headed naga guards the entrance.

The cave has been formed by water erosion and allegedly has an underground corridor that connects all the way to the Mekhong River, several kilometers away. Visitors are let in only in small groups of a few people at a time,  as most of the area in this subterranean place is narrow while the ceiling is mostly low.

Year-round, the cavern is at least in part inundated and visitors are asked to take off their shoes before entering, as  in most places one needs to walk through shallow streams or pools of still water.

Whereas walking upright would be virtually impossible for adult visitors most of the time, in a few places one will also have to squeeze through narrow corridors, often while wading through low water.

At certain spots the passage is in fact so narrow or low that visitors will have to crawl on their knees or belly in order to pass through.

The naga tunnel cave is eventually exited at the top by a series of steep ladders, just a short walk back downhill to the main entrance where visitors can retrieve their shoes.