The Tam Cung Grotto or Three-palace Grotto is situated in the centre of Ha Long Bay, 5 km from Sung Sot Grotto to the northeast. The grotto itself is divided into three parts; after threading the stone cracks, you find the first chamber. Many beautiful forms can be garnered from the rocky shapes.

Continuing on to the second chamber, the bumpy way leads to a spring with fresh water. The last room contains further incredible stone depictions.

Tam Cung Grotto lies in May Den, a luxuriant island nearly isolated from other islands. May Den’s cliffs are extremely vertical, while its forest is flourishing.

Entering its first chamber, one can hear the sound of t'rung or stone musical instrument from near and far. Pushing further inside, tourists meet a "fairy" whose white beard flies in the wind climbing the mountain. High in the wall of the chamber there stand three statues representing prosperity, happiness and longevity, looking down at the earthly world. From the ceiling falls down a stalactite which resembles the bud of a hydrangea among a garden of stone flowers. Going through a narrow passage, one comes to a natural museum of lively animals, including lions, seals, even god of the sea. In the middle of the second chamber lies a stream murmuring all year round, making the two surrounding cliffs rustle. In the last chamber, tourists can find an imposing bas-relief in which are carved elaborate strange images, lying layer after layer in a harmonious layout. One can figure out flowers, bamboos, stone curtains, or sleeping elephants.