Halong Bay, a UNESCO World Heritage Site, is Vietnam's leading tourist sight. A karst formation has left a dispersed group of nearly 2 000 islands: rocks of different shapes and sizes covered with saxicolous plants break the surface of turquoise waters, where caves and inland lakes lie hidden. A paradise for pirates in the late 18C, the bay is now popular with tourists, who, under restrictions imposed to protect the bay, follow a prescribed route.