Today"s photo brings us to the banks of Caño Cristales, the "liquid rainbow" that cuts a prismatic path through the heart of Colombia. From June through November, when the clear water is low, the abundant underwater plants that cover the riverbed show off their red, yellow, green, and blue hues. The star of the show—especially in this photo—is Macarenia clavigera, a riverweed that ranges from bright red to deep crimson or purple depending on its intake of sun rays. In August and September, when the florid flora is at its peak, the river"s a coveted nature excursion: Because the ecosystem is so fragile, visits are limited to guided tours.