Sai Kung's hexagonal columns are the wrong colour. Nearly every famous colonnade on Earth is dark basalt, but these are pale, silica-rich rhyolite, exposed nowhere else on anything like this scale. They date to about 140 million years ago, when a supervolcano here emptied roughly 1,300 cubic kilometres of ash and collapsed into a caldera. The ash pooled inside the hole, welded into solid rock, then cooled so slowly and evenly that it split from the surface downwards into columns averaging 1.2 metres across, the broadest reaching three. The sheet runs up to 400 metres thick across some 100 square kilometres of land and sea, and in 2022 the International Union of Geological Sciences placed it on its first list of 100 geological heritage sites. At Po Pin Chau, a stack the waves cut away from the cliff behind it, the columns stand 100 metres above the water.
Only one stretch can be reached on foot. Excavating the East Dam of High Island Reservoir in the 1970s sliced the cliff open, and a short geo trail now runs along the cut, close enough to lay a hand on the hexagons: one wall bends into an S, folded while the rock was still soft, and a boardwalk drops to a sea cave that thuds with each swell. The dam's ocean face is armoured with thousands of heavy interlocking concrete blocks, piled there to break the swell of the typhoons that regularly cancel the boat and kayak trips from Sai Kung town. You cannot drive yourself in: the access road is permit-only for private cars, so visitors come by taxi, by the weekend minibus, or on foot. After dark the reservoir holds the darkest sky in Hong Kong, and the Space Museum's Astropark, Hong Kong's first, sits at its other end.