As a kid, did you ever scribble a random shape in black crayon, fill in the gaps with vibrant colour, and call it art? That's Cano Cristales, a five-coloured river in La Macarena, Colombia. Every year, from September to November, the water flows slowly enough to reveal a dazzling living rainbow created in part by a brilliant red plant called Macarenia clavigera. Other hues come from the variously coloured quartzite rocks in green, black, blue and yellow.