Class VI (Extreme). Waterfalls, water thundering through jumbled chutes choked with rocks and steep drops—this one will make your knees weak. A Class VI is the outer limits of boating. Once, Class VI rapids were considered unrunnable. Better boating skills and technical advances in equipment, such as self-bailing boats, have allowed some previously unrunnable rapids to be rafted by teams of experts. No commercial rafting takes place in rapids of this caliber. Take pictures from shore; just don’t get too close to the edge. (Yahoo Factor: Silent prayer.)