Caño Cristales is a river in the Serranía de la Macarena of central Colombia that for a few months each year runs in five colours at once, its bed carpeted in blazing crimson by a plant that flowers underwater. The plant is Macarenia clavigera, a riverweed found only here, which grips the bare rock in fast water with fine red fronds that comb the current; it turns red only when the water drops enough for sunlight to reach it, so the river burns brightest between flood and drought, from late July through November. Then the crimson and magenta lie under clear water, with yellow sand, green plants, blue pools and black rock between, and the river looks painted. The rock it runs over is quartzite laid down about 1.2 billion years ago, far older than the young Andes rising to the west, and the water has scoured it into smooth basins and potholes that you can slip into on a hot day.
The river is walked. Guides lead along the rock beside the water, stopping where the plant blankets the bed deepest, where the river spills over ledges in low cascades that pour straight onto the red beds below, and where the pools are deep enough to swim, with the only rules being no sunscreen and no touching the plants. Crouching by the shallows you can study the thing behind the whole spectacle, a plant a few centimetres tall that blooms while submerged. For most of living memory none of this could be seen: the river lay inside territory held by FARC guerrillas and off-limits to outsiders, and it opened to visitors only as the war wound down, reached by a flight to the small town of La Macarena and then by boat and on foot. The number of visitors is capped, and the river is closed for half the year to let the plant recover. Caño Cristales is called the river of five colours and the liquid rainbow, and for once the nicknames are accurate.