It Dam Bac Islet is in the front of the grotto and Soi Sim Islet in the back.

A narrow way leads to the interior of the grotto where a stream flows. The stalactites here are snow-white and lightly hang from the ceiling.

is linked to the ancient legend of the Golden Tortoise: after having helped Emperor Le Loi defeat his enemies, the Golden Tortoise took back the magic sword he had given him, and swam towards the sea. Arriving in Halong Bay, it met with so many evil spirits and demons hindering its journey that it remained to do battle. After having defeated them all, the Golden Tortoise was so exhausted that it searched for a grotto to rest in, and once inside, turned to stone.

Today in the grotto, it can be found dozing with ancient wounds covering its body. 

Source: vietnamtourism.com