The grotto of Tam Thanh situated in Tam Thanh Ward, Lang Son City, Lang Son Province, is one of the famous grottoes in Lang Son, lying west of Ky Lua Street. Its door is approximately 8m high, opening to a corridor with 30 stone steps carved into the mountain. There are dense groves hiding the grotto from the sun. Poet Ngo Thi Si (1726–1780) carved one of his texts on the grotto wall while he was a Mandarin guard in Lang Son.

The poem praises the magnificent natural beauty of the area, more specifically the sound of the water stream falling on hundreds of rocks, almost like a graceful murmur. Vong Phu Mountain, meaning “waiting for a husband,” is located nearby. Here, one can explore a grotto with marvelous stalactites and a statue of Buddha.