Refer:
The karst seascape of Ha Long Bay is one of the world’s most spellbinding sea views and is a UNESCO World Heritage Site. With the bay’s scenery best seen by boat, this is prime cruising territory. You should opt for at least an overnight tour to see Ha Long Bay’s iconic views as a day trip doesn’t do it justice. There are plenty of caves in the bay that can be entered including the Hang Sung Sot, with three mammoth caverns, and the Hang Dao Go, with superbly weird stalagmites and stalactites. For most people though, the highlight is simply cruising amid the karsts and soaking up the changing scenery of pinnacles as you pass by.
