At Trang An, in northern Vietnam, the rivers do not go around the limestone towers but straight through them, and a flat wooden sampan takes you in with the water. You sit low, almost at the surface, and the boat slides into a cave mouth and on under the hill, stalactites a hand's width overhead in the cool dark. Minutes later the far end opens on another valley of rock and green water. The main loop links about nine of these flooded tunnels, with old temples tucked against the cliffs between them. It is the same limestone as Ha Long Bay, a hundred kilometres away, except that here it stands on land, in a maze of rivers, lakes and flooded paddies.
The other route, down the Ngo Dong at Tam Coc, trades the caves for open country, the boat sliding between walls of rock and flooded fields. The rower is worth a minute of your attention. Many here loop the oars under their feet and push with a slow leg stroke, hands resting in their laps under a conical hat, because hours of rowing by hand wreck the back. The paddies gild twice a year, around May and June and again in autumn. At harvest the fields burn gold right down to the waterline, often under a collar of morning mist. For the view from above, 486 stone steps climb a karst ridge at Hang Mua to a carved stone dragon laid along the summit, with the river coiling in silver loops through the fields below. The same ring of rock was once a capital. At Hoa Lu two low stone temples mark where the Dinh and Le kings ruled a newly independent Vietnam a thousand years ago, with the peaks around them doing the work of city walls. The court left in 1010, judging the valley too cramped, and the walls it left behind are the ones you row through.