Iguazú Falls is not one waterfall but 275, strung along three kilometres of cliff in the rainforest on the Argentina–Brazil border, and at its centre the river gives way altogether into an eighty-two-metre chasm called the Devil's Throat. Nothing here comes down in a single drop. The cliff is a staircase of old lava, so the water arrives in steps and in dozens of separate falls, with green islands and standing forest in between. The Devil's Throat is where that politeness ends. On the Argentine side a long catwalk runs out over the upper river, flat and quiet and almost dull, until it reaches a railing, and a step beyond it the whole river goes over the edge. Below is a horseshoe of white with its floor lost in spray, and from there the noise takes over everything.
Iguazú is two national parks, and each gives you what the other cannot. Argentina puts you inside it. The Lower Circuit brings you almost level with the foot of several falls, close enough for the spray to come over the railing and for a rainbow to stand in it on a sunny morning. The boats go further still, pushing upriver from below until the pilot noses straight into the base of Salto San Martín, where the engine vanishes under the noise, the white wall closes over your head and you come out soaked through. Brazil gives you the whole line at once, along a paved trail on top of the far gorge that stays with the falls for their full width and ends with the Devil's Throat filling the view. Away from the boardwalks, the Macuco trail runs three and a half kilometres of dirt into closed forest, one of the last surviving pieces of the Atlantic Forest that once ran the length of Brazil's coast. And there are birds that live behind the water. Great dusky swifts nest on the wet rock behind the curtain, so every adult has to fly through the falling water to get home, and every chick makes its first flight through it.