COLOMBIA // For an enchanted window between July and November this otherwise ordinary river transforms into what looks like run-off from a crayon factory. Blankets of underwater river weed bloom scarlet, the colour contrasting with the yellow sand, black and green river rocks and blue water. In the Sierra de la Macarena National Park, only 200 visitors are allowed daily. Protect the ecosystem by swimming only in designated areas - no sunscreen or bug spray allowed.