Diving can be categorized into two types: professional and leisure. Professional diving mainly refers to those diving activities performed by experienced professional divers, and includes underwater projects, underwater salvage, and underwater adventure. Leisure diving is designed for underwater sightseeing, leisure and entertainment, and is classified as snorkeling or scuba diving (i.e. diving with an air bottle and an underwater breathing apparatus). Diving to sightsee has become a common leisure activity in seaside resorts.