Nestled on the periphery of Vietnam’s fabulous Halong Bay region, rugged, craggy and jungle-clad Cat Ba, the largest island in the Bay, is emerging as Vietnam’s adventure sport and ecotourism hub. There’s an energetic roll-call of activities, including sailing, bird-watching, cycling, hiking and rock climbing.

Except for a few fertile pockets, Cat Ba’s terrain is too rocky for serious agriculture. Most residents earn their living from the sea, while others work in tourism. In recent years, Cat Ba Town has experienced a hotel boom, and a chain of ugly concrete hotels now frames a once-lovely bay. But the rest of the island is largely untouched, and with idyllic Lan Ha Bay just offshore you’ll soon overlook Cat Ba Town’s over-development.

Most of the year, Cat Ba Town is a laid-back place, and an excellent base for activities around the island, or sailing and kayaking around Lan Ha Bay. On summer weekends Cat Ba turns into a roaring resort, filling up with vacationing Vietnamese. Hotel prices double or treble and there’s an excess of karaoke joints and hubbub. Cars are banned from the promenade, which is taken over by a sea of strolling holidaymakers. Weekdays are saner, but still busy between June and August.

Three beaches are located near the harbor town – hardly world class, but they’ll do for a quick swim and some baking on the sand. A national park is a good place for a close-to-nature trek among the mangroves and freshwater lakes and it’s a good launching point for tours of Halong Bay and kayaking trips.

Almost half of Cat Ba Island (with a total area of 354 square kilometers) and 90 square kilometers of the adjacent waters were declared a national park in 1986 to protect the island’s diverse ecosystems: subtropical evergreen forests on the hills, freshwater swamp forests at the base of the hills, coastal mangrove forests, small freshwater lakes and coral reefs. Most of the coastline consists of rocky cliffs, but there are some sandy beaches and tiny fishing villages hidden away in small coves.

Lakes, waterfalls and grottoes dot the spectacular limestone hills, the highest rises 331 meters above sea level. The island’s largest body of water is Ao Ech Lake (3 hectares). Almost all of the surface streams are seasonal. Most of the island’s rainwater flows into caves and follows underground streams to the sea, resulting in a shortage of fresh water during the dry season.

Cat Ba’s best weather is from late September to November, when air and water temperature is mild and the sky is mostly clear. December to February is colder but more pleasant. From February to April is still good, but you can expect some rain. Summer (June to August) is hot and humid with occasional thunderstorms. This is also peak season and the island is packed with Vietnamese tourists.