One of the most visited caves in Halong Bay, accessible from a dock about 6 km from Bai Chay. The cave extends about 250 metres into the limestone and contains impressive stalactite and stalagmite formations — columns, cascades, and shapes that Vietnamese guides interpret as dragons, phoenixes, and celestial dancers. The coloured lighting is a Vietnamese cave standard that some visitors find overwhelming.
Thien Cung is almost always included in cruise itineraries. If you're going independently by day boat, it's easily combined with a visit to the Dau Go cave nearby (larger, with more raw geological character). The crowd density inside depends heavily on how many boats have moored at the same time — cruises that dock at different times of day have better luck with thinner crowds.