Some 2-3 km southeast of Trongng Mai Islet lies Bo Nau Grotto or Pelican Grotto. This vaulted grotto covers 200 m2. The floor of the grotto is wide and flat, but not deep, and its wall features lots of stalactites and stalagmites.

At the entrance are three stones, looking like three fairies with their heads close to one another. Two men appear to be playing chess, with the third serving as a referee.
Bo Nâu Grotto is one of the beauty grottoes in Ha Long Bay.

The name Bo Nau or Bồ Nông is associated with the fact that pelicans often take shelter here. If Ðầu Gỗ Grotto lies completely inside the island, Bo Nau opens its mouth at the side of the island. Stalactites fall down from the roof of the grotto in different clusters. Meanwhile stalagmites grow up in different shapes and layers. Is this that the life out there is so noisy that the three fairies have to choose this tranquil place to play chess? Turning southward, Bồ Nâu Grotto enjoys cool wind. Standing there, one can hear the murmur of the sea all year round.