Cat Ba Archipelago lies 60km from the city centre by sea and covers 345km2, formed by 388 limestone islands large and small rising from the water. Rolling limestone ranges interspersed with lagoons and bays, magnificent caves, and outstanding biodiversity, geological and geomorphological value have made Cat Ba Archipelago a masterpiece of nature. It was recognised by UNESCO as a World Biosphere Reserve (2004), ranked by the Government as a Special National Scenic Landscape Relic (2013), and became part of the Ha Long Bay - Cat Ba Archipelago World Natural Heritage Site (2023). It is also home to more than 3,000 species of terrestrial and marine flora and fauna, many of them listed in the Red Books of Viet Nam and the world. Notably, the Cat Ba Langur is an endemic species found nowhere else on earth.