Cat Ba is the largest of the 366 islands spanning 260 km² (100 sq mi) that comprise the Cat Ba Archipelago, which makes up the southeastern edge of Ha Long Bay in Northern Vietnam. Cat Ba island has a surface area of 285 km² (110 sq mi) and maintains the dramatic and rugged features of Ha Long Bay. It is commonly used as an overnight hotel stop on tours to Ha Long Bay run by travel agents from Hanoi.