Vietnam's Caves and Halong Bay: The Country's Most Dramatic Landscapes

Vietnam's geology is dominated by karst — limestone dissolved and sculpted by water over millions of years into towers, caves, and islands of extraordinary variety. The two most famous expressions of this geology are Halong Bay in the north, where 1,600 limestone islands rise from the Gulf of Tonkin, and the Phong Nha-Kẻ Bàng National Park in central Vietnam, which contains the largest cave system on Earth.

Halong Bay is one of Southeast Asia's most visited destinations, and the challenge is finding a way to experience it that rises above the crowded day-trip boats. The answer is to book a two or three-night cruise on one of the better-quality junks, which allows access to the less-visited areas of the bay — Bai Tu Long Bay to the northeast, or Lan Ha Bay to the south — where the limestone islands are just as dramatic but the other boats are far fewer.

The best cruises include kayaking through the sea caves and floating fishing villages that are inaccessible to larger boats, and the experience of paddling through a narrow cave opening into a hidden lagoon — the water perfectly still, the limestone walls rising 100 metres on all sides — is one of the finest in Southeast Asia. Dawn on the bay, when the mist lies between the islands and the light is soft and grey, is the moment that justifies every other inconvenience of the journey.

Phong Nha-Kẻ Bàng National Park, in Quảng Bình Province, contains more than 300 caves, including Sơn Đoòng — the largest cave in the world by volume, large enough to contain a 40-storey skyscraper and its own weather system, with clouds forming inside the cave and a jungle growing in the light shafts. Access to Sơn Đoòng is strictly limited to 1,000 visitors per year and costs around $3,000 per person for a four-day expedition.

For those without the budget or the booking lead time for Sơn Đoòng, the Paradise Cave and the Dark Cave offer extraordinary experiences at a fraction of the cost. The Paradise Cave is 31 kilometres long and contains formations of such scale and beauty that it has been described as a cathedral of stone. The Dark Cave combines a zip line over the river, kayaking, and swimming through a mud pool inside the cave — an experience that is simultaneously absurd and genuinely thrilling.

The town of Đồng Hới is the base for exploring Phong Nha and is easily reached by train from Hanoi (six hours) or Hue (two hours). The national park is 45 kilometres from town, and most visitors hire a motorbike or join a tour. The road through the park, along the Son River, is one of the most beautiful drives in Vietnam — a valley of limestone karst, jungle, and river that feels entirely removed from the rest of the country.

The best time to visit Phong Nha is from February to August, when the dry season keeps the cave trails accessible and the river levels manageable. Halong Bay is best from October to April, when the northeast monsoon brings cooler temperatures and clearer skies. Both destinations are worth visiting in the same trip — a two-week itinerary that combines Hanoi, Halong Bay, a train south to Phong Nha, and then Hue and Hội An covers the best of northern and central Vietnam.