Adventure travel is a type of niche tourism, involving exploration or travel with a certain degree of risk (real or perceived), and which may require special skills and physical exertion. In the United States, adventure tourism has grown in recent decades as tourists seek out-of-the-ordinary or “roads less traveled” vacations, but lack of a clear operational definition has hampered measurement of market size and growth. According to the U.S.-based Adventure Travel Trade Association, adventure travel may be any tourist activity that includes physical activity, a cultural exchange, and connection with outdoor activities and nature.