Divers face dangers that can hurt or kill them if they don't know what to do. They can use up all their air, or get decompression sickness or nitrogen narcosis. A person must be trained to use the equipment and dive safely. When they show that they can dive safely they get a certification card. The biggest organization for certifying divers is PADI - Professional Association of Diving Instructors - but there are many others, depending on the country. Some tourist places have a short course without certification and then the instructor will lead the diver in a shallow dive, all in one day.