Cat Ba is the largest island in Halong Bay. It boasts tropical evergreen forests, freshwater swamp forests, coastal mangroves, freshwater lakes and waterfalls, grottoes, caves, sandy beaches, offshore coral reefs and a few small fishing villages. Cat Ba is a beautiful island. Some 20 species of mammals roam the island, including boars, deer, hedgehogs, monkeys and squirrels and the waters around Cat Ba teem with arthropods, dolphins, fish, mollusks and seals. There are over 70 bird species on Cat Ba, as well as nearly 800 different plant species.

In 1986, more than half the island was declared National Park. Cat Ba National Park is an effort to protect the island's ecosystems. Cat Ba Island is 30 km off the coast from Haiphong and excellent for hiking. You should bring a flashlight if you want to explore any of the island's caves. The most famous cave is Cai Beo Cave, one of the 17 places on Cat Ba Island where stone tools and bones from around 4500 BC have been found.