The sea and island zone of Quảng Ninh has a unique terrain. There are more than 2,000 islands, making up two-thirds of the country's islands. They extend along the coast for more than 250 km and divide into numerous layers. The largest islands are Cái Bầu and Bản Sen, while the smallest are just like rocks in a garden. Vân Đồn and Cô Tô are island districts. There are thousands of islands on Hạ Long Bay and Bái Tử Long Bay, many of which contain caves. Besides areas of alluvial deposits, there are also white sand zones which have built up from tidal action. Some places have formed white sand mines producing materials for the glass industry (Van Hai), or have developed into tourism spots (Trà Cổ, Quan Lạn, Minh Châu, Ngọc Vừng).