The Great Smoky Mountains are not officially listed among the "Seven Wonders of the World," but they are recognized as a UNESCO World Heritage Site and an International Biosphere Reserve, which are high-level global honors. Their "wondrous" status comes from their incredible biodiversity; the park is home to over 19,000 documented species of plants and animals, making it one of the most ecologically diverse areas in the temperate world. Geologically, they are among the oldest mountains on Earth, formed 200–300 million years ago. While they may lack the specific title of a "Wonder," they are often described as a "wonder of nature" due to the unique "smoke" (blue-tinted mist) that blankets the peaks, which is actually a chemical vapor emitted by the dense vegetation. As America’s most-visited national park, they are considered a natural treasure of global significance.