Located 9 km west of Cam Pha, Quang Hanh Grotto is the longest grotto in Halong Bay. It is 1, 300 m long, and stretches throughout the stone mountain of Quang Hanh. The French named it "Le Tunnel, " or Tunnel Grotto.

Quang Hanh Grotto is accessible by either boat or car, but the entrance only appears when the tide is out. Ba Co Shrine (shrine of three girls) is in the grotto beside a smooth stone block. Legend tells that three girls, who were once journeying on the sea, came to the grotto to take shelter from the rain. They were so engrossed with the beauty of the grotto, that they did not notice the rising tide. They drowned, only to become water goddesses.

Quang Hanh Grotto is extremely beautiful. A small boat will take you through the stone passageway by flashlight, casting magical colors on the hanging stalactites.