Río Celeste runs an opaque sky blue through the rainforest of northern Costa Rica, and it is not blue at all until one exact point in the forest. That point is Los Teñideros, the dyers. Two clear streams come tumbling in out of the trees, join, and within a few metres the water clouds into turquoise, all inside one pool. Nobody could explain it for over a century. The answer came in 2013, and it turns on the size of the particles. One stream is acidic enough to make the aluminosilicate grains carried by the other clump together, growing until they are the right size to scatter blue light back at your eye. It is the same physics that colours the daytime sky. A glass of it comes up clear. The blue exists only at river scale.
The trail is about six kilometres through dripping forest, loud with frogs and birds and with howler monkeys working the branches over your head. A long wooden staircase takes you down into the gorge to the waterfall, where a narrow ribbon of white water falls into a round pool the exact colour of the sky, and nobody is allowed into it. Further along, a short climb reaches a mirador above a still blue lagoon with the cone of Tenorio behind it, the one place the sky-blue water lies calm instead of rushing. A suspension bridge crosses high over the river, and from the middle of it the turquoise slides through the forest under your boots. At a stretch called Borbollones the river seems to come to the boil: volcanic gas rises through the riverbed in strings of bubbles and the air turns sharp with sulphur, which is the Tenorio volcano breathing under the forest. For the Maleku, whose valley this is, the river has always been sacred and there was never any riddle to solve. When God had finished painting the sky, they say, he rinsed his brush here.