Tam Thanh Grotto 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. Here, one can explore a grotto with marvelous stalactites.

Poet Ngo Thi Si (1726 - 1780) carved one of his poems 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. 

Situated inside the Grotto is Tam Thanh Pagoda with a statue of Buddha. Visit the vestige on the 15^th day of the first lunar month every year, visitors can join the Tam Thanh Pagoda festival with rituals and traditional religious customs of Lang Son’s people.