The extraordinary, mossy limestone mountains that rear from the waters of Ha Long Bay form one of the most spectacular natural landscapes on the planet. At least 1,600 green islands rise over a flotilla of strung-together fishing boats and floating villages. Spectacular at any time, sunset bathes the remarkable limestone formations in warm, honey-coloured light, adding an extra layer to the seascape's heart-stirring beauty. Emerald green sea washes around this immense network of islands and limestone outcrops, which you can explore at leisure on junk boats and by sea kayak. Local legend says the islands were formed when a dragon descended, spitting fire and emerald and jade jewels across the water to deter invaders. The sheer scale points to the supernatural – but scientists stubbornly maintain that this collection of towering sculptures was the result of various forms of erosion and a flood of seawater following the ice age. The water continues to chip away at the islands, and you can venture into the mouths of caves hollowed out by the relentless wash of the waves. Dau Go Cave and Sung Sot Caves are adorned with rows of spectacular stalactites, descending like dragon teeth from above. Investigate via boat, and find island jewels like Ti Top - a sharp, slope of land decorated with a crisp fringe of gleaming sand. The seaplanes that soar overhead, offer a magnificent bird's eye perspective. Located in Vietnam's northeast, the unique topography that forms the islands continues into neighbouring national parks like Cat Ba - which rises and falls in a similar, jaggedly undulating fashion, just without the flooded seawater in-between.