Situated in the centre of the UNESCO World Heritage area, Sung Sôt or “Surprise Cave” is on Bo Hòn Island, and is one of the most spectacular and largest grottoes of Ha Long Bay. The mouth of the grotto is reached by ascending about a hundred stone steps. One is surrounded by trees and foliage on the journey up as the shimmering bay below offers an ever more panoramic vista to admire.

The cave is partitioned into two chambers; the first being similar to a wide theatre hall, with many stalactites hanging from the high ceiling. A narrow passage leads to the second chamber, where a flow of natural light bathes the surfaces. The light is filtering in from above, through a large opening creating a natural skylight, this opening also serves as the exit from the grotto. The chamber is so immense it could contain thousands of people at one time.

At the deepest point of this chamber a "royal garden" appears with a clear pond and a fascinating landscape of small mountains. Many birds and plants (benjamin figs, cycads and centenary banyan trees) can be seen here. On some days groups of monkeys straying in from the dense vegetation of the island might appear, in search of fruit.

The grotto covers some 10,000 m2. There are thousands of stalactites and stalagmites along the 500-meter paved passage. Light posts line the passage and illuminate the amazing scenery. Spotlights of varied colors are placed in such a way to be unobtrusive and add to the enchantment of the grotto.

The 30-meter high roof of the grotto, the walls and innumerable cracks and crevices are evidence of millions of years of spectacular natural creation. Close to the exit a mammoth formation stretches from floor to ceiling, tapering at the center to form a concave pillar that appears to be keeping the ceiling in place. Walking through the cave ones imagination can run wild conjuring up all kinds of fantastic creatures and fanciful creations.

And of course, as always here in Vietnam, there are legends! One such involves a rock which seems to form the shape of a horse and a long sword. As the legend goes, Thánh Gióng (Saint Gióng) helped the people of the area to chase away evil spirits and demons. After this feat, Saint Gióng flew to heaven, leaving a stone horse and sword behind to guarantee the demons did not dare to return.