Somoto Canyon is a slot cut into welded volcanic ash in the Madriz department of northern Nicaragua, near the Honduran border, four metres wide in places under rims of 190 metres, where the longest river in Central America, the Río Coco, begins, and it is waded, swum and floated in inner tubes for three and a half kilometres of narrows with guides from the villages at its mouth, who found it for the world in 2004. The stone can be dated to the eruptions of the Miocene, but the canyon is young, cut by the river in the last few thousand years, and the round basins in the rock underfoot were bored by pebbles spinning in the current; the route starts on dry thorny hillsides, works down over boulders to where the pools grow too deep to stand in, and then the walls rise, so that the visitor rides low in a wooden boat or swims the green pools between potholed bars of pale water-sculpted rock, with the guides stopping at rock shelves above pools deep enough for a jump, from a modest step to a drop of several metres. In the dry months the river runs low, cool and clear green, and the pace is the swimmer's rather than the current's.
After heavy rain the river lifts several metres and turns the colour of milky coffee, when the narrows are floated rather than swum, and at full flood the authorities close the monument. The Río Coco born here goes on to divide Nicaragua from Honduras for most of its 750 kilometres, and the two countries spent half a century arguing over the border it makes, settled by the World Court in 1960. The canyon was known to the farmers of the valley and to nobody else until two Czech geologists surveyed it, and the families of the hamlets at its mouth run the trips, with lunch in their yards after. Somoto is a crack that a river starts in, and the river that leaves it is the longest in the isthmus.