Peat swamp forest is a strange kind of wealth. Because the water sits high and sour, dead leaves break down slowly, and the undecayed remains build upward year after year until the forest is standing on a mattress of itself. Tràm — melaleuca, the paperbark tree whose trunks peel in pale sheets — dominates the canopy, its roots gripping a substrate that is closer to compost than to earth. Between the stands, the open ground carries móp, năn and reeds, plants that tolerate acid water and long inundation. Cà Mau province holds a great deal of Vietnam's remaining mangrove, but this is something else entirely: freshwater forest, inland from the coastal mangrove belt, growing on carbon it made itself.

Protection came late and in stages. The core of this forest was a wildlife sanctuary known as Vồ Dơi before the prime minister signed Decision 112/QĐ-TTg on 20 January 2006, raising it to national park status and giving it the name U Minh Hạ — Lower U Minh. The park's strictly protected core runs to a little over 8,000 hectares, spread across communes in what were then the districts of U Minh and Trần Văn Thời. Around it sits a buffer zone of more than 25,000 hectares, three times the size of the park itself. Three years later, on 26 May 2009, UNESCO recognised the forest as one of three core zones of the Mũi Cà Mau Biosphere Reserve, the designation that ties this inland peatland to the mangroves and mudflats at the country's southern tip.

The species list reads like an inventory of what a delta can still hold when a few thousand hectares are left alone. Hairy-nosed otters hunt the channels. Pangolins, deer, long-tailed macaques and wild boar move through the tràm; turtles, snakes and pythons work the margins between water and root. Birds and insects fill the canopy, and the ditches carry freshwater fish in numbers that would surprise anyone who thinks of the Mekong Delta as one continuous rice field. None of this is accidental. The peat holds water through the dry months, and the water holds everything else.

Ask the people who live around U Minh Hạ what season matters and they will not talk about tourism. They will talk about the first heavy rain. When it arrives, the forest floods, and the flooded forest becomes an enormous nursery — fish spawning through the drowned undergrowth, eels moving into the shallows, the year's protein arriving on schedule. Households here make their living from the forest directly: keeping bees for honey, fishing, catching eels, tending fruit trees at the forest edge. That dependence cuts both ways, and it explains something visitors often miss. These families are fiercely protective of the forest, not out of abstract conservation sentiment, but because a bad year in the tràm is a bad year at home.

U Minh Hạ has a sibling. Some seventy kilometres north, across a provincial line that has itself been redrawn more than once, lies U Minh Thượng — Upper U Minh — protecting the same peat-and-melaleuca system. Together they are the surviving fragments of a forest that once ran almost unbroken across the base of the Cà Mau peninsula, a place with a long reputation for swallowing outsiders and sheltering whoever needed sheltering. What remains is small by the standards of what was lost. It is also, in the flat and heavily farmed delta, close to irreplaceable.