In a nocturnal rendezvous, two green tree frogs meet face-to-face atop a leaf in Louisiana's Atchafalaya River Delta. Green may be the most common color found in nature—it's everywhere from leaves, grass, and moss to snakes, butterflies, and even the northern lights. Green represents life, vitality, nature, and, of course, environmentalism. A pool of water at the mouth of Rio Frio Cave reflects the greenery of Belize’s Chiquibul Forest and frames a visitor in silhouette. The easily accessible river cave is a popular attraction. Two boys share a nighttime bicycle ride down the darkened streets of Hoi An, Vietnam. The city's old town is a World Heritage site filled with historic buildings from the 15th to 19th centuries.