The summer Milky Way appears dazzling in dark starry sky of the Yellowstone National Park, a World Heritage Site in Wyoming, USA. Note the green cast above the horizon which is natural emission of the Earth upper atmosphere, known as the airglow, visible colorless to human eyes under dark skies. Down on the Earth is the Kepler Cascades; a waterfall in the Firehole River. Preserved by the government since 1872 the Yellowstone is the world's earliest established national park, known for its wildlife and geothermal features.