Río Celeste runs through Tenorio Volcano National Park, and at a point called the Teñidero two clear streams join and the water abruptly turns opaque turquoise. The explanation is not dye or minerals in solution but optics: aluminosilicate particles suspended in the water reach a size at which they scatter blue light, and the effect only works in sunlight, which is why the river can look ordinary and grey on a wet morning.
The trail from the ranger station runs about three kilometres each way past the waterfall — a sixty-metre drop into a pool of that same blue — the Teñidero, the borbollones where gas bubbles up through the riverbed, and a hanging bridge. It is steep in places, very muddy in the wet season, and rubber boots are hired at the gate for good reason.
The park is reached from Bijagua, about an hour and a half from La Fortuna and two from Liberia. Go early: entry is capped daily and the light is best before the afternoon cloud.