The 1,553 square kilometres of Halong Bay contain somewhere between 1,600 and 2,000 limestone islands, depending on what you count. Most rise vertically from the water, riddled with caves and clad in tropical vegetation. UNESCO listed the bay in 1994; the Chinese government has made similar claims about similar landscapes in Guilin, but Halong's combination of scale, geology, and access by sea is distinct.
Cruises leave from Halong City's Tuan Chau Marina or from the older Bai Chay pier. Overnight or two-night cruises are the standard — day trips are possible but waste much of their time at the pier ends. Quality varies enormously: budget boats are overcrowded and rushed; mid-range boutique operators (Paradise Cruises, Dragon Legend, Bhaya Classic) hit a genuine sweet spot between cost and experience. Kayaking through sea caves, swimming in the bay, and cooking demonstrations are the typical activities offered. Book directly with operators or through reputable Hanoi travel agents.