Somoto Canyon looks like limestone country and contains no dissolved rock at all. The slot runs three and a half kilometres through the Madriz department of northern Nicaragua, four metres wide where it pinches, with the rim as much as 190 metres overhead. The rock is welded volcanic ash that will not dissolve, so everything here that looks karstic was cut mechanically. The round basins underfoot were bored by stones: caught in an eddy, a pebble spins in one spot and grinds its hollow deeper with every flood. Water alone would barely scratch this stuff; add grit and gravel and it becomes a drill. Farmers had fished and washed here for generations, and nobody outside knew of it until Czech geologists walked in during 2004.
The way in comes off dry, thorny hillsides and down over boulders, until the pools grow too deep to stand in. A guide is waiting there with a wooden boat, and you sit low in it while the walls close overhead and the sky narrows to a bright ribbon. What the day feels like is decided by the river. In the dry months it shrinks to a chain of dark green pools divided by ramps of pale, water-sculpted rock: you clamber over the bars and swim the gaps, and the water stays cold even when the hills above are baking. The guides stop at ledges above pools deep enough to jump from, anything between a modest step and a drop of several metres, and the cold of water that rarely sees direct sun tends to make the decision for you. After heavy rain the river lifts several metres and turns the colour of milky coffee; then the current does the carrying, and at full flood the place shuts. The villages at the mouth take turns supplying the guides. The river they hand you back to is the Río Coco, longest in Central America, which goes on to divide Nicaragua from Honduras for most of its length.